I stand corrected. It's not restricted to House Mercere.
But it would appear to be restricted to those with some Mercere nexus: House Mercere, Cult of Mercury Societates within Flambeau or the Neo Mercurians.
There's enough shading to say that the mastery ability isn't widely known. And when compared to Ceremonial casting, it is mentioned in Mysteries that it is a widely known mastery ability.
They're available to members of either the Cult of Mercury or the Neo-Mercurians, or to someone who was expressly leaked the secrets. That's not the same as "any magus whose lineage is even remotely Mercurian." There's plenty of Guernici and Tremeres (for example) with Mercurian Magic who are not, and never were, part of Priamitus' Cult of Mercury and don't have access to that bucket of magical secrets.
The Flambeau section of Societates gives a big bucket list of Virtues & Flaws that qualify a magus as "remotely Mercurian". The whole thing is an excuse as to why a magus in play at the time the book was published might not have them in their core design.
I'm aware of that bucket list on pg. 17, but possessing one of those virtues does not make you a cult member. It's a list of suggested virtues that are particularly fitting for a cult member, and at the end, it mentions that cult members have access to the Spell Mastery virtues at the end of the chapter. You don't have to have any of them to have access to the Mastery virtue, you can just say "my parens was in the Cult and taught me the mojo."
In my personal sagas (warning, House Rules territory incoming), I allow all magi with Mercurian Magic to use the Spell Mastery abilities in question. The Cult of Mercury has a Minor Breakthrough by Priamitus that integrated the Mercurian Spell Masteries into regular Hermetic practice, but they remain a cult secret (for example, the Terrae-magi don't have them), and a magus will have to be an active member of the Cult and swear to keep the secret before they will be given access to the Hermetic spell masteries. Ceremonial Casting, of course, leaked at some point and is now regular Hermetic practice.
Marko, and many others, for that matter, play lip service to the Cult of Mercury stuff.
Lip service is all it really deserves. Look at it this way. The original set of spell masteries were written up and published in the core book because that is all they had thought of (and tested) at that time. The rest were invented later by different authors. What was needed was an excuse, however flimsy, for why characters already created and in game might not know these new masteries. And provided an easy way to gain them.
Fiction, Fiction, Fiction.
That is all it is. What fiction best fits your personal aesthetics.
I myself allow players access to most masteries with a tenuous excuse, if even. Really, I do not even care. Some GMs like to be restrictive. I take the opposite approach.
If interested, I had years ago created Spell Mastery reference and HRs for my saga. Have a look at them here. Most of the new ideas were introduced as an experiment, and most have never been used. If I were to revise the list< I would trim it doen and toss out many of the ones no one ever picks.
Plus I always looked at Mercurian as a trait of Lineage, not an actual Cult.
I always looked at Mercurian as meaning a lot of different things, including an active Cult - probably because I happen to actually like the idea of said Cult, with varying degrees of devotion (Cult of Mercury as a sort of traditional homage, and then the actual daimonists - who are likely to seek out the Neo-Mercurians).
Obviously, your sagas will vary.
Sure. You have a game that puts House Flambeau in preeminence, they have a mention in HoH:Societates about a nexus with the Cult of Mercury, just shave a bit and it works. For you. In your saga. It's not RAW, not that RAW is well defined in this regard, and it's a matter of setting, so to each saga their own.
With regards to the tenuous excuse bit, my pbp saga started with a magical Mercere, it makes sense that the Cult of Mercury mastery abilities be somewhat exclusive. It gives that character something of an edge, something to trade, something to negotiate, and the ability to do something that no one else can do. One could say that the opposite approach makes the value of Merceris Magi in your game significantly less. Seemingly simple decisions like this can invalidate entire concepts, or make them look cookie cutter, at the very least. I wouldn't want to play a Merceris Magus in your saga, because I don't feel that they have anything to add to your saga.
The irony here is that very few players actually choose Flambeau, and the most frequent option by far is a Mercere magus. So much so that I had to make adjustments to account for it. 20 Mercere magi in 1242 as opposed to a dozen in 1220. Over the past ten years, there have been about 4 or 5 PC Flambeau, and as many as 8 magi Mecere and about as many Redcaps.
I totally disagree with your interpretation of RAW, but that is okay. Nothing new there. Instead, I am debating you on results, which have been the very opposite of what you theorize.
Mercere magi are a pain in my book simply because of their extreme popularity (because of their utility) contrasting with their rarity. So I don't particularly care to give them any more than special Redcap privilege and Mercere's portals.
The Cult of Mercury is something not specific to House Mercere, just common there - it's also common in Flambeau, and may have adherents in Bonisagus, Guernicus, et cetera (anyone but Tremere or Diedne, or probably Bjornaer or Criamon).
Oddly, my results look a lot like Marko's even if I laugh at his specific interpretation. Cult membership doesn't mean much except that you expressly identify yourself with the particularly Roman roots of the Order of Hermes.
Hey, you suggested I play a Mercere magus when I joined your saga. 
I do not remember that, but it sounds like something I would have done. The whole Mercere quarter was a long established part of the covenant, and having an NPC Mercere in charge would suck.
But I also recommend Flambeau to everyone. Few take up the challenge.
Hi,
I don't think it even deserves lip service.
Why should someone get a free Virtue "Lots of Cool Masteries" just because? (Or other magi be saddled with an extra flaw, if you prefer to see it that way.)
Insofar as I'm concerned, all masteries that do not require a virtue should be available to everyone. I similarly ignore "only House X has this spell with a guideline that doesn't require a virtue." Just utter yuck.
(Then again, I prefer to think of the Mercurians as fictitious. They are as mythological in Mythic Europe as Jupiter (the god, not the planet) is in ours.)
Anyway,
Ken
I agree with that. I can't see why some character backgrounds should be rewarded with extra goodies, while others get nothing. If there were some parity with backgrounds, that would be one thing. Having different Houses get different bonuses is generally okay, for example, because there's some parity in that. Everyone belongs to a House and presumably gets some benefit from their House. But for someone to be able to say that their character concept warrants them to receive significant bonuses whole other concepts don't seems unfair.
But then I've gone and used what one of my law school professors called the four-letter F-word. "Fair."
Why should one character be compelled to perform a season service for their House year? (Tremere) The setting answer is that the House provides them support in exchange for that.
A lot of the mastery abilities aren't all that great, either. Stalwart is pretty great, to be sure. But the idea is fairly consistent, if poorly executed, getting access to additional "power" requires some sacrifice, see above with Tremere, and then there's the whole Cult of Mercury Societas. The idea is that if you want some extra power, you have to fulfill some obligations, somehow, and they're described as being fairly light.
Oh,
I have an issue with the automatic lost Tremere season too, though it is supposed to be totally balanced by extra House support and resources. I suspect that both aspects are handwaved in many sagas.
Anyway,
Ken
House Tremere, unlike your covenant, has copies of the best available Roots for every Art (they traded the use of their junior magi for them), along with several Branches that they managed to get off of Durenmar. In fact, they probably copy said Branches internally at Coeris - while they don't sell copies of said Branches, they do make them available to more Tremere magi than they would be at Durenmar (which has an unofficial policy of not actually making their good books available). They pay handsomely for a library of quality tractati. No Tremere magus should have to study from vis.
House Tremere has longevity specialists on contract. (No, this isn't specifically on the list of available gifts, but come on.) When a Tremere magus turns 35, they can be assured that their exarch has already marked them on one such specialist's calendar and that their LR is paid for.
Coeris has a bigger stockpile of magic items than anywhere except Verdi, tailored to a variety of uses. Need one? Just sign for it. (Youngsters, expect to wait a bit.)
In short, House Tremere is a lot closer to that ideal Order that works together and shares resources for the good of all than...well, the regular Order is. I would assume that Tremere magi can count on decent compensation for the season they spend in service to the House.
Ramirez, that's your saga. And while I agree with it, it's not in place in all sagas.
To be honest, it's OK to let some character backgrounds get goodies as long as those goodies cost something. In the case of skill masteries, those masteries require xp which have to come from somewhere. To get lots of skill masteries would require you to take Flawless Magic, which uses up your slot for a major hermetic virtue - so preventing you taking Mercurian Magic or the Tamed Magic at character creation. Letting Tremere get extra stuff in exchange for service is fine, as long as the two are roughly proportional. So your other character can't start with some particular bonuses - there are a huge number of ways of achieving mastery in your chosen sphere of magic, so you could instead focus on more Arts, or tweaking your lab, or joining a Mystery Cult, or adventuring for a lost magic secret, or.....the list is infinite.
Your magus will never be able to do everything. If the range of backgrounds means someone might be slightly better at one aspect than yours, decide whether you want to specialise in something different or ask your SG for an adventure that will let you gain access to something that will compensate. Darius of Flambeau does not care who has access to strange skill masteries or mystery cult virtues - he cares about whether he gets to be a Hoplite, and getting enough Creo, Ignem and Penetration to hunt the evil-doers down.