It seems like you can avoid having your unwrangled fandom be invisible in Yuletide if someone posts a drabble or other short fic to AO3 (not to the Yuletide collection) ASAP. The tag gets wrangled once a fandom appears in the main collection, and then it becomes canonical for Yuletide. This isn't guaranteed to work, but it did seem to for some people on the last coal post.

Coalies, are you writing a first-time or otherwise unwrangled fandom? Say its name and a line or two about what it is, and someone can write a drabble! (And then probably orphan it since odds are it was written from Wikipedia.)