Hi,
You got it! The institutions able to support this are already in place, canonically.
As for House Mercere, when I wrote the stuff about "if you sell 10 copies, you get..." I imagined the possibility of House Mercere paying you 10 seasons worth of fp up front for the right to all future sales which shall be at that set price, allowing you to retain one copy for yourself. House Mercere breaks even upon selling 10, and looks forward to centuries of profit, as well as cementing their critical importance to the Order: Sure, if you have decent ReCo or CrIm or various other magics, you don't need Redcaps for snail mail, but they're also Amazon, and you always need books.
The beauty of this is that once you buy into the idea, you don't need to worry about any of that unless you want economic fluctuations to be the major theme of your saga. Just set fp to be what you want it to be (I do think Marko nailed it with qp, and this works for any Tribunal), and assume that the Order is large enough to buy any books your players want to write on Arts, MT, Finesse, Penetration, Parma (in sagas that allow such books) and Mastery for common spells, and that the Order is large and old enough to supply any books you want at the standard price.
No population models needed.
If need be, handwaving that the extra vis comes from the rich lands of the far off Novgorod and Theban Tribunal (or Hibernia, if you're on the other side of the map) works fine too, and might even inspire a story involving a long journey to those fabled lands.
You might want to create rules for Reputation of writers though. Good writers get a Reputation for that, but if you publish many books, everyone has a pretty good idea of what you know. And if you think your players are reading too much more than writing, it's easy enough to have some NPCs slight the covenant at Tribunal because they aren't really participating in the Order.
Anyway,
Ken