Yep, the biome token I ended up using was to specify ANY_LAND if I recall, since "found villages in any region, even the most extreme climates" was the intent behind that idea.
Meanwhile, I'm pondering a more perplexing thing. If you have secret interactions, you can control who's valid to obtain them to a degree. You can not only screen for vampires and werebeasts, and ensure they're intelligent, but more importantly for what I'm doing I can tell the interactions to check for SYN_CLASS. This is what ensures things like picking up the War sphere's version of the "go apeshit" spell prevents you from picking up a redundant version of the spell from Strength, for example. It also lets me add spell opposition, like making it so that learning the healing spell prevents you from learning blight magic.
However, I'm ALSO adding syndrome-causing items that players will be able to craft from megabeast hearts and related materials. This is an issue, as there seems to be no equivalents to IT_REQUIRES and IT_CANNOT_HAVE_SYNDROME_CLASS. So that means I seem to be unable to make the megabeast-derived spell-granting items as selective as the secrets are.
Would I maybe be able to make it work by assigning a SYN_IDENTIFIER to each spell type? My idea here is that if all variants of the spell-granting syndrome have the same identifier, will this ensure that the syndrome CE_CAN_DO_INTERACTION effects won't trigger a second time?