Yes, I seem to recall a bit like this. You may declare, and they take champions. It is rare, though, iirc.
Generally speaking, this is why Redcaps don't get too uppity. It is easy to smack them down, because they automatically lose certamen. It needn't come to Wizard's War, and in fact that's worse for the magus because then the Redcap can do sneaky things like poison his mail and he is likely to get House Mercere upset. Now Loukas is indeed too uppity, and I think it is fair to say House Mercere would not have any problem with a powerful magus handing him a big embarrassment. So I have an idea for what happened.
Let's say as part of their Iberian investigation, Pythos and his assistant Loukas encounter resistance from a Quaesitor at Duresca, Punctuatus of Guernicus. He is a relatively young Quaesitor, about Pythos's age, and has recently taken on a new Hoplite, Equetus of Flambeau, since his previous Hoplite (Guillaume) was injured in battle. Punctuatus believes the investigation is over, as he got a lot of acclaim for being the one who wrapped it all up, and he seems to feel Pythos is trying to undermine his success by continuing the search. He refuses to meet with Loukas, which further arouses their suspicions, so Loukas decides to poke around the Quaesitor's sanctum, and see what he can turn up. He discovers several books and lab texts written by one of the infernalist magi that suggests the two of them knew each other. He can't steal the books, unfortunately (that would be depriving him of his magical power), but what he does do is he hides them somewhere else in Punctuatus's sanctum, like under a floorboard. Then he reports back, and Pythos considers what to do with this information.
Soon after, Equetus and Punctuatus show up at Doissetep. Punctuatus is ostensibly there to check on Guillaume (but they suspect he is looking for the missing books), and during their visit, Equetus is as pugnacious and provocative as he can be to Loukas and Pythos, goading them and insulting them publicly, until Loukas finally punches him in the face. Equetes smugly challenges Loukas to certamen, framing the matter as "You will never show your face in my presence again, or in the presence of my sodalis." Loukas haughtily agrees, with the rider that if he wins, if he does see Equetus again, each time he gets to punch him in the face once more. Equetus rolls his eyes and says "Creo." Loukas nods and says "Vim." Equetus nods and begins to draw the circle. Loukas takes his place. Equetus is puzzled, but proceeds to summon the magical phantasms. Loukas makes a big show of pulling down his sleeves, doing absurd gestures, and babbling in a combination of Greek and Latin "magic words," and thankfully the phantasms on his side of the circle appear.
The duel is fast and brutal, since Equetus is off his guard and furious once he realizes what is happening, but he refuses to yield. Both combatants shout insults and jibes at each other, but it is clear that Equetus is tiring, while Loukas is not at all affected. Finally, Loukas's figures overpower Equetus's figures, and the panting magus falls unconscious, venomously calling out Pythos's name as he slumps. From his sanctum, peering through his intangible tunnel, Pythos decides that no matter what, this Flambeau magus is now an enraged and dangerous enemy, and so he elects to cast, as his Creo Vim spell, a spell that gives Equetus enough Warping Points to immediately send him into Twilight (since he cannot resist it). The Flambeau magus's body disappears in a dreamy swirl of colors, leaving behind his robes and other possessions. Obviously startled, Loukas stands, saunters over and presses at the pile of clothes with his foot, and pulls out a sword. "I'll just keep this for him until he gets back," he says to the gathered onlookers (including Punctuatus, who leaves without a word).
Perhaps this event causes Guillaume to confront Pythos, as he is probably aware of what happened. He might be angry at how Pythos treated another Hoplite and fellow member of Guillaume's House. Perhaps Pythos tells him about his investigation, and about what he has learned about Punctuatus, but that he is not sure how to act upon it. They discuss Punctuatus, what they know about him, and decide that they should investigate him more carefully together, possibly separately so that the Quaesitor does not know what they are up to.
When Equetus returns from Twilight, he might immediately declare war on Pythos, if you want to play out a full-blown Wizard's War. Pythos could always send him into Twilight again, for longer this time. However, it might be that Equetus instead accepts his defeat and nurtures his grudge against Pythos. And it could be that Equetus already had a lot of Warping, and is gone for quite a long time.
What do you think?
I'm not sure they could "cheat" like that. I mean, they could, but there would be a case for prosecution.
But pythons could easily have been Loukas champion, with the exact same results.
I figure Loukas could just claim Pythos was his champion anyway. In the story, it was clear (eventually) that Pythos was fighting Equatus on Loukas's behalf, right? Besides, I bet the assembled magi really enjoyed the fight. Even with Loukas's enemies, his "refusing to respect certamen" would be really hard to make stick. 
Well, you gotta declare your champion before. Imagine. You're facing a magus known for his mastery of creo and herbam. Luckily, you know he is weak in vim, average in rego, and manage to corner him into these 2 arts, in which you are stronger. Save that, unbeknownst to you, you're instead facing his tremendous pal who happens to be a vim monster.
You challenge one magus. And you know who you face, which can be. Really, really important. He may choose a champion, but given a good enough OoH lore, you probably know him too.
Having this kind of trick sprung up on you? It's just cheating. And that's assuming you can fight certain through an IT, which IMO is absolutely not possible.
So yes, pythons as champion 
This is a cool story, and I can imagine the gathered onlookers cheering style before substance. And I can also imagine that Loukas announced a champion right at the very last moment, and the opponent was too proud to back off.
Sure, so long as it was another Quaesitor, Dilgentia, who had assigned us the job. (Part of Guillame's command structure?) This isn't a task that Pythos would have happily took up on his own, and he is looking to be part of team.
If he grabs an Arcane Connection to the books, then Pythos can examine them without ever needing to step into the sanctum.
While I think everyone would get a good laugh at the chubby Criamon stepping in publicly and legally, I would really like it to have gotten all the way up to Wizards War, and plus there's a certain sense of cut-and-dry, black-and-white if that happens which I'd like to make more murky...how about:
Equestus challenges Loucas to Certamen. Loucas accepts, calling for a champion as is his right. A voice speaks up magically, announcing that it will champion the Prince...and then there's a deflating pause as Pythos heaves his fat @$$ up the stairs to the confrontation (that probably causes quite a laugh). Equetus grossly underestimates Pythos as Just An Intellego Guy (a reputation that Pythos would pretty rightly earn early on in his youth) - so he calls Rego, not Creo, gets royally trounced, and at the end of the fight Pythos casts his Touch/Moon Duration version of I-Tunnel on him.
Later, Equestus is murdered. Punctuatus accuses Pythos of doing it, with his Intangible Tunnel. Pythos counters with accusations of diabolism. The case is tainted in the eyes of those around us by the fight, Punctuatus manages to explain his possession of the texts satisfactorily to the judges, but not to us. We are not allowed to take part in the investigation of Equestus's murder, but magical investigations bear no fruit.
A covenant-mate of Equestus, Salamandrus of Flambeau, convinced that we were guity for his death, declares Wizard's War on Pythos. Minutes after the next full moon rises over the walls of Doisstep, Salamandrus disappears in a puff of magic before he has even finished his first incantation -- it seems that in his anger, Salamandrus forgot that Pythos had stayed at their covenant earlier that year, and had never revoked his Aegis Token.
Pythos would be surprised at an angry response. According to his Criamon faith, he acted in a very Apt fashion. He did not engage in violence, nor even harm his foes. Instead, he blessed them with the opportunity to experience the embrace of "Twilight" (as the Order calls it). But perhaps this doesn't even happen until saga start, and we can RP it out.
Again, the point of the suggestions above is to make what actually happened and who is responsible for what all the more murky. I'm not married to any of it, but I like the general nature of the conflict.
How about Dilgentia, at Magvillus, assigned Pythos to investigate the infernalism privately, since the official investigation has declared the case closed? Punctuatus is annoyed that an outsider seems to be butting in, unaware that he is actually sanctioned as part of a special team?
Debatable, I'm afraid. True Lineages treats books as a Group of pages, so it could be that you'd need an arcane connection to each individual page to read the whole book, or at least a way to turn them from afar. Anyway, that's too easy, isn't it? I think it's much more interesting if Pythos can't get his pudgy hands on the books, and it leaves it ambiguous exactly what secrets they contain. Maybe there's a Peripheral Code ruling that says copying a book without permission is tantamount to taking the original book, and that scrying on them is just like copying them? And while Loukas can get away with poking around in someone's sanctum without committing a High Crime as long as he doesn't use magic to do it, using your Intangible Tunnel definitely counts as scrying, doesn't it? Presumably the Quaesitor can discover if any magic was used in his sanctum and figure out who did it and what type it was?
Aw, this isn't quite as fun as Loukas pretending to engage the guy in certamen, but I suppose he could milk "summoning his champion," which might make Pythos's entrance seem a little more impressive. 
Pythos or Loukas could veto Creo, of course, which might lead the Flambeau to choose Rego instead. If it was Pythos and not Loukas declaring Vim, though, I am certain Equetus would veto it. I thought the idea of Loukas kind of forcing his boss into certamen was kind of amusing. The scene after that would be especially enjoyable.
That's hardly an impressive victory for Pythos... 
Could we just say that Equetus is really angry at having been embarrassed by Pythos in the duel, and hotly declares Wizard's War against him? Perhaps he doesn't intend to actually go after Pythos, just avenge his honor with another challenge, but of course Pythos wouldn't know that, and so his blasting him into Twilight is a pretty impressive offensive strike. Makes a great story. I guess I'm asking, do you need a Hoplite to die to get Pythos involved in this war? Equetus might be a better enemy if he has more history with us (and I really want the opportunity for Loukas to punch him in the face again). You could certainly rename him Salamandrus if you like.
Excellent! He'll be even more pissed at you afterwards 
I do agree with mercuria, though, that it'd be better if it was Equestus that's send into sparkyland
This wouldn't happen to be the same Salamandrus of Flambeau who used to live at Phoenix and took Claudia as his apprentice before they blew up his lab and destroyed her gift? (I doubt it, but...)
I'm good on that. Maybe Punctuatus knows we have "sanction" from someone who he doesn't think should be butting into his area, or maybe he doesn't know we've received orders at all.
On the other hand, TMRE provides rules for the S&M bonuses and target numbers for opening a book as a greater invested device or talisman. And, it should be pointed out, Pythos has an Obsession(minor) with Scrying, and pretty regularly violates the legal definition with abandon. He's like a hacker working for the CIA.
Counter-Intellego work is a really valid argument, on the other hand.
If you want to add more drama to the story, please feel free to do so. What I was trying to add was mystery and confusion, without losing the elements you're looking for.
Only if you know that he had the Aegis token
The point of including the Wizard War in Pythos backstory is to emphasize the Criamon point of view -- to Pythos, blasting him into Twilight is a really gentle, non-violent response.
I'm not well-versed in "legitimate" reasons to go to Wizard's War. I need to find time to read the HoH:S an HoH:TL sections on the subject again. More thoughts on this later -- I gotta go write a novel.
I mean, it'd be funny if it was, and I don't object to the crossovers, but I just use Salamandrus as the generic name of Flambeau since the one you mention.
I want to explore the potential tie between Loukas and Carmen, 1212 when she was at Juno's Spire. He seems like the sort of bad boy that young Carmen might have had a fling with. Older Carmen maybe not. But I look forward to playing out that moment, "...so, we meet again..."
Definitely sounds amusing.