All great suggestions, thank you! I explored the built in PreParse event, and it probab;y would have been where I put this stuff if I had been using STM from the get go in my development. As it stands, I have a significant amount of code devoted to building/validating/massaging text before it gets to STM, and it makes the most sense for it to go there.
I checked out Konash's solution, and at the end of the day, it's very similar to mine. It maps directly to the glyphs of final ligature forms. The big difference is that everything is hard coded in their solution, while I'll be needing different ligature sets/mappings for each different language in my game.
I DO think it would be possible to make a truly generalized solution to this, but that would be a project unto itself. Ultimately it would look very similar to either my code or Konash's, but it would build its mapping tables by opening and consuming the OTF files directly.
Ultimately I am just flabbergasted that Unity has not implemented this and has no plans to. Ligatures are... just fundamental.