I’m looking over the playbooks, and the Idealist’s Alchemist ability makes reference to formulas for poultices and talismans.

Sample formulas don’t appear to be listed in the text, nor a process for inventing them. I assume the intention is for individual groups to invent their own, perhaps taking inspiration from the Blades ritual &/or recipe rules—is this correct?

Yes, that's precisely the intent!  I imagine that some sample formulas would be useful: we're hoping to include some in the upcoming expansion zine: Serpent & Oak.