The question was: "Does anyone know a good solvent for masking tape
adhesive, that won't eat plastic keys?"

Try a product called "Goo Gone".  Grocery and hardware stores have it.
It's a scented liquid hydrocarbon-based solvent for removing label
adhesive.  I have even used it to remove chewing gum residue from auto
carpets.  You might wish to try it on a plastic scrap to make sure it
won't discolor the surface, but I have never had that problem.

Steve McCollum