As a habitual player of Ex Misc (or merinita), I thought the census is one of the more interesting points of canon "current house politics."
In my current game with CJ23 as alpha SG, the census is something we keep being threatened with - my character (an ex misc from a tiny, doomed, but powerful tradition native to Britain) and another character (skilled with Herbam magic, forests, and either has a spell for everything or is strangely good at spontaneous magic) are very resistant to having outsiders poke their noses into our little traditions, which we are trying to pass on to the next generation. In fact, we're suspicious that someone would try and use the information to cull our lineages at a later date. As such, and as the other Ex Misc has become a quaesitor and has somehow found a staff suspiciously like Pralix's, we are working the politics and trying to lay the foundations for a coup, with me pretending to be fiercely loyal to the current primus.
In the game I'm playing on the boards here with Peregrine Bjornaer, my character is a Pralixian who's met a quaesitor obsessed with punishing everyone. As such, he would find a list of different Ex Misc magi and any information on their magical specialities a treasure trove to help him meet wizards with new kinds of magic, and registering magi would be a great way of protecting them from quaesitorial suspicion, so he is keen to support the census.
As to how to do the census - the quaesitors are pushing for it, in order to stop rogue magi and magi orbi from claiming "but I'm an ex miscellanea". I would use the quaesitorial support to help push it. Send messages via the redcaps to every Ex Misc that you're aware of, and ask them to register their magi and those they've trained and pass the message on. Meanwhile, get the quaesitors to announce at each Tribunal that all Ex Misc magi in the tribunal should send their names in for registration, and if they can't pass the message on to Cad gaddu they can always give the information to House Guernicus in strictest confidence. Use the quaesitors to encourage people forward. Of course, magi from covenants that don't send representatives to Tribunal, who aren't known to other Ex Misc magi in the tribunal, and don't regularly correspond via the redcap network may end up missed out. Such magi should make the best of their miniscule presence, because if their actions attract the attention of quaesitors they could be in trouble. Of course, not being in regular contact with Tribunal decisions means they could be at risk of ignoring legal decisions that affect their behaviour anyway, such as not realising a limit on magically created wealth has become part of the peripheral code in their area.
How big is Ex Misc? I usually assume bigger. There's always some tiny tradition someone's forgot to count, or Mythic Companions who aren't gifted but have been offered membership in the Order as part of Ex Misc. More importantly, there's probably someone who's being deliberately missed out - whether a Mystery Cult so mysterious they insist on not being mentioned, or people whose powers are so awesomely freaky no-one wants to let on what secret weapon their covenant has in reserve.