There are several options the PC "magi" might pursue, especially if their players are well read in Ars Magica. I recommend to check these out before starting "the Big Bluff".
(1) A very promising and well hidden concept is turning into RoP:F p.118f Homunculi Wizards (see also LoH p.82f Caerfloron). Some Faeries might be very much attracted and nourished by the "Big Bluff" story they participate in.
(2) Forging pacts with powerful spirits (see RoP:M p.88f Spirit Votaries) might be another, likely less promising option.
(3) The obvious challenge for the PCs is avoiding offers for RoP:TI p.124ff False Magic, especially The False Gift, before they find some safer acceptance in the Order without it. Deceptions maintained for a long time are an open invitation to the schemes and forces of hell. But playing this up too much can destroy your saga quickly and easily - so handle with care.
One quick note in case I was not clear: the basic premise of the saga is not about finding some way to acquire the Gift (though I did describe one of the possible PCs as having that goal): it's about the challenges of PCs bluffing their way as a covenant of "real" magi.
An earlier version of the concept was about the covenfolk of a covenant long in winter still dominating hermetic politics despite the last of their magi having passed into final twilight 50+ years before. But that is somehow easier, as you depend on the pretense that some powerful (and eccentric) magi who really did exist and impact hermetic politics not too long ago are still around. In the BIg Bluff, instead, you have completely mundane PCs that must pretend to be individual magi.
Beyond the obvious need to fake a modicum of magical ability, at a minimum, I think this group would need:
An item to harvest / store vis;
Some method of faking magic resistance (relics, guardian angels, etc.);
The active complicity of a redcap covering their tracks - because if they depend on a redcap NPC to carry out their errands, they are the most likely to figure out that the magi covenant is importing items for its every need, and never needs to acquire vis for its projects somehow.
I also would suggest at least one supernatural ability per character, because otherwise, they'll end up looking like Verditius magi who happen to depend on buying items from Verditius for their effects.
The main advantage you will have (short term) is the prohibition of scrying on other magi. A major difficulty will be covering your tracks for acquiring items to fake your advancement, because if every covenant member looks like a junior magi for decades, someone is bound to start asking questions unless they're all secluded in such a way that no news leaks out. And visitors from the order are likely to notice something is strange when there is no Aegis giving them a tingling when they visit. I'm not sure how realistic it is to keep up the deception long term. A junior covenant could justify not having the resources for an aegis. But I just don't see any magi finding it normal that there has been a lack of an aegis for the past ten years at a junior covenant that is exporting vis somehow.
At least a few probably would ... though I think it's more about the "premise" so to speak: how did they get to a) bluff their way until joining the covenant and b) learn about the other "magi" in the first place?
That depends a lot on the nature of the covenant's vis. Though I agree that, eventually, it would be useful, and not that expensive, to have such item(s) made.
Again ... why? While it would certainly make the disguise easier, I do not see it as necessary.
Again,, while a redcap helps a lot, I do not see such a character as strictly necessary.
First of all, the covenant need not import every item from the same redcap, or even the Mercere network. It can just trade with other covenants. Second, even if a redcap imports every item the covenant needs, is he going to figure out those are the sole items the covenant has? Third, and this ties up with the previous point, maybe the covenant is dedicated to some fairly focused and specific endeavour, and it has all the vis it needs for that specific endeavour-- and it imports everything else.
Well, it's not that a magus is constantly slinging magical effects around, and even if he is, it's not necessarily clear whether they come from spelllcasting, enchanted devices, bound creatures, inherent powers etc.
But this is a good point. In general, for such a covenant magic will be precious, precisely because the PCs are UnGifted. Now, if I allow every PC to have supernatural abilities, it will end up being a covenant of UnGifted hedge magicians... which is a different, and much less extreme kind of bluff to pull off. So no magic for the PCs!
This creates a problem though. What about grogs/specialists? My troupe is sneaky, and if I say "no magi, and companions cannot have supernatural abilities" I am pretty sure I'll get a host of grogs with Second Sight, Premonitions, Enchanting music, Mythic Profession (covering all the Forms, at two Forms per grog) and perhaps even magic grogs with much stronger powers. So ... I'll probably need some covenant-wide reason why any character inherently tied to some supernatural realm just cannot be around, at least not for long. Some curse or prophecy, or maybe just some ill-tempered sleeping dragon that is allergic to supernaturally-aligned beings? Ideas would be welcome.
And more in general, magic for all needs. One big thing that comes to mind are longevity rituals, though there are probably alternatives.
Well, you know, there are ways to explain why the magi refrain from flashy magics. There may be a mystical reason - whether they have become very pious, or they are onto some big project that requires all their puissance so they can't afford to use it otherwise. Or perhaps they can Jerbiton their way into being the arbitres elegantiarum of the Order, and convince everyone that big flashy magics are sooo last millennium and subtletly is the new black.
That's quite true. But note that any item pinging visitors who enter will have the same effect as an Aegis.
One major point I am having trouble with are credible reasons for all the PCs to be part of the same big bluff. I mean, how can they have learnt of each other's secret, if they are all so very careful about not letting that secret slip out?
One possibility is that some ... mystical power, perhaps the same that keeps supernaturally-aligned characters out, managed to draw the PCs to the place. Then again, it must be somewhat discriminating to be able to specifically draw in (what is probably) Mythic Europe's entire population of fake Hermetic mages, rather than random mundanes. Perhaps people versed in Order of Hermes Lore have a reason to seek the covenant's site out, and the mystical power has rebuffed all but the PCs? Strained, but it could work.
Another possibility is that one of the characters is indeed absolutely, phenomenally good at Intrigue, and has managed to ferret out others like him ... figuring out it's much easier to hide such a secret together with other people who have it rather than alone?
One idea is that they are closely related family of some sort. Could be Sibling, or Brother-in-Law, Sister-in-Law... Potentially adoptees into a family. maybe even inter-PC marriage. Another thing, is that people need a reason to not expose the others - and that can be something they are running from.
Probably the best thing they can have is a mutual outside threat- i.e. They all need to take some variation of Enemy, Fued, etc to encompass different aspects of the same unifying outside threat.
Believability. Magical Air simulates the effect of the gift in nearby mundanes. A group impersonating a magi covenant that doesn't have Magical Air would be seen as a covenant of Gentle Gifted. Magi expect other magi to trigger such reactions of distrust arround them, and Gentle Gift is an exception. While you can fully have a covenant built arround the Gentle Gift, bear in mind that the less the covenant as a whole seems like normal magi, the less likely their impersonation will work in the long term. Of course, if the concept of the covenant plays on using its gentle gift for the benefit or the order, that may actually help them, for a while. Until the first disguise falls...
Again, believability. If every spell that hits them penetrates, that puts them at risk of discovery. It only takes one apprentice that casts a prank spell at a social gathering that appears to penetrate without effort to blow up the cover of a 40-year old magi, when other magi wonder how it is possible for him to be so incompetent that an apprentice can penetrate parma + form resistance. If none have magic resistance, this charade might work a few seasons, maybe a few years, but it won't survive a few decades. You should expect at least one player to be uncovered. And if a character is uncovered while his sodales are known to treat him as a full magi, how plausible do you think a magi that discovers a fake magi will not investigate to see if this covenant is genuine, if they are also known for not having an aegis of the hearth, and they are all gentle gifted? It takes merely one disguise to fall to put the entire covenant at risk.
I suppose you could create an item with a creo imaginem effect with penetration that could fake this tingling, yes. Not that any third party crafter would probably consider an item made to detect approach and react by an imaginem effect that fakes the tingling felt when approaching the aegis to be highly unusual to say the least, and making that order could bring complications in itself.
Oculus Septentrionales does really all to impersonate mundanes in Lübeck, including forbidding those without the Gentle Gift to join the covenant and check them out - plus they betrayed their original mission.
But there are still no rumors around, that they simply are mundanes impersonating magi in the Order.
... which just is the obvious fall of any case of the OP saga concept - no matter as what kind of magi the PCs posed. Of course in the Oculus Septentrionalis case, then there would be lots of magi who always knew.
Sure. Just to clarify, I don't think a covenant of gentle gifted is suspect in itself. What I think, is that if one of the characters impersonating was ever identified as a non-magi, then unless the person who makes that detection is more interested in stuff like blackmail, then the plausible outcome is a quaesitorial investigation to find out whether the rest of the covenant knew and helped in that impersonation (potentially causing several crimes in the process, such as a non-magi voting, etc.). There are situations where the discovery of one of the impersonators could, in a quaesitorial investigation, not destroy the whole cover of the entire covenant. I just think it's almost impossible that the quaesitor is not thorough in his investigation if the entire covenant is known to be gentle gifted, weak with magic resistance, and having no aegis of the hearth. In other words, my arguments is that the less elements they have that coincidentally make them appear like a typical covenant of magi whose arts are progressing through time, the more likely an investigation will bring the entire charade down, rather than merely one of them.
Really? If he is not a magus, a Quaesitor would just read his mind and ... Bingo! He has the other impersonators as well.
EDIT: There can be of course covenants consisting of magi without the Gift who are perfectly legit in the Order: Mercer houses full of un-Gifted Red Caps, covenants of magi who have sworn their Oath and then lost the Gift, and such. But we do not talk of these here, do we?