Is there anything that specifically says anything about rituals and masteries? To practice a mastery, you have to cast it multiple times across the whole season. I get the feeling this would be an incredibly bad idea when you factor in Vis costs, so are there even books on ritual mastery abilities?
In addition, can anyone join the Cult of Mercury with little repercussions? The mastery abilities listed look really good (especially Adaptive Casting...) so I was wondering what the catch is.
There isn't really a catch to joining the Cult of Mercury - there's a reason that most SGs handwave the whole thing to a greater or lesser extent.
To the extent that there is a catch, it's that you need to identify yourself with the Mercurians. You might be asked to host a Mercurian ceremony now and then, and that a large portion of House Flambeau (the Mithraians) hate your guts. It's a loose societas at best.
On mastery, I see no reason why a magus could not practice all of the steps he would take in a ritual without actually casting it. This would mean no expense of vis. I'd probably insist that the magus cast the ritual once per season of practice, but once would be enough.
Remember that a Flawless Caster automatically Masters any spell they know, and that Aegis of the Hearth is one of the most common spells in the Order. Other Ritual spells in the core rules (such as Shrouded Glen, permanent healing spells, and Conjuring the Mystic Tower) can also be Mastered in this way, and supply and demand suggests tractati on them have accumulated over the centuries.
In my saga, Flawless Casters are the nucleus of a consortium of magi who produce books on Spell Mastery (and also Casting Tablets, which can only be made by a magus who has Mastered the spell), with House Mercere taking orders for tractati from magi around Europe, then offering these contracts to the Flawless Casters, who can then bid on the contract. Contracts that don't get picked up quickly (probably because the spell is rare, so none of the Flawless Casters know it) can be re-offered at a higher price, effectively paying the Flawless Caster to first invent the spell and then write the Mastery book.
HoH:TL, talks about Fenecil's Rituals on page 76. It talks about how they are practiced and the associated masteries. Not the same thing, but close enough?