There is nothing in the rules preventing or allowing it, thus we enter the area of House Rules.
The way I would rule it as ST is: if the story is good and it does not become a tool for munchkinism, I will allow it, even encourage it - after all, if a player comes with a project his mage would like to do and it makes good story involving all players, he is doing half of my job 
, it deserves a rewards.
From a story side, The Mysteries describes how someone could actively look to (re-)create a cult/Mystery, starting by building a Cult Lore then discovering initiation script. It is also conceivable that the reverse happen: that somebody is looking in a given obscure topic, get some insight (simulated by the acquisition of a new virtue) and then just realises that he stumbled upon an old mystery cult. Then as a ST, I would also grant him a new skill - New Mystery Cult Lore: +1 (or more depending on the story background). Unknowing to himself, as the mage was looking into this obscure topic, he was reconstructing some secrets of a forgotten cult.
If you want to make it even more interesting, what the mage discovered was not about an forgotten cult, but about an existing one - still unknowingly to him. The kind of secret that the existing Cult would not like to be spread outside his members (think of Bjornaer Inner Mystery of Sensory magic, Verditius Inner Mystery of Automata, etc...), for example the ability to initiate a unique virtue, without Ordeal requiring to take an Oath of Obedience or a Vow of Secrecy.
And because the mage discovered the secret on his own, it might be closer to the founder vision, without the various layers of interpretation that were added over several centuries of subtle "tweaking" by the various leaders either because they believed it was better, or to fulfill some personal agenda. For example, what if the first script did not required taking an Oath as the founder wanted to spread this knowledge, but was added afterwards to "strengthen" the cult - so the knowledge would only be shared to obedient follower ? A bit like somebody finding the 10 Commandments and deciding to live by it, without any knowledge of the Bible, Old or new Testament, then discussing innocently about his way of living and how it seems that Fate smiles to him. He is following God's words (with likely limited level of awareness), but I am sure that some priests would see some form of heresy and would rush to "bring back into the flock" this misguided soul because he does not know anything about the Church, the tithe, obeying the priest and all that very important stuff to be a good Christian.
Anyway, to come back to this magus, he might be triggering a Schism between the tenants of the current Cult and the ones looking for going back to the roots, the "good old days". Which, by the way, might not be so "good". What if the changes brought after the founder were real improvements and following the old ways might slowly draw the practitioner in the wrong path (Chtonic magic slipping into Devil worshipping for example) ? Think of a Cult that Guorna would have been following (or created)...
A peaceful Merinita, looking at the original work of the founder, finding some interesting pieces of information, acquiring some new powers/virtues, calling it "Cult of Gaïa" (or any other peaceful-sounding name), whereas in fact, he stumbled on Diedne initiation scripts... Or maybe the real Cult name should be "Followers of Echidna" (Greek "mother" of monsters).
Plenty of opportunities for stories.
And because it is house rules, you can use Initiation rules as a guideline to see if what the mage is undergoing looks appropriate, eyeball it and run with it, without keeping an accurate tally, at least for the first few initiations. Don't forget to give him some appropriate character traits. If this knowledge has the potential to push the person to dig deeper, in more forbidden area, the trait might increase (Obsessed with Cult or Loyal to the Cause), or you might decide to use a mechanism similar to the Verditus Hubris: it strengthen his ability to perform Cult magic, but it also drives him to behave in a certain way. In the case of Verditius, it is about being competitive and proud about their achievement. For something darker, the level in this ability might requires mandatory actions, more and more demanding as the skill increases (odious habits, then eating living animals, then sacrifice...).
It might not be as sinister, just odd. Let's go back to the Merinita looking into Cult of Gaïa: maybe the early requirement is to sleep on soil and walk bare feet, then she has to live naked, then to eat only raw food and finally to forfeit the use of human speech. And maybe that was the "Good Old Ways" of the Cult and the current version allows to achieve the same powers, but instead of having this requirement, it was replaced by other, more socially acceptable ordeal and flaws.