After reading the interesting and insightful information on this very old thread (https://forum.pdpatchrepo.info/topic/6185/feedback-fm-algorithm) I realized that perhaps I could achieve the desired effect through a little sleight-of-hand.

Just use a "clone" of your operator! In theory, this should work for an arbitrary wavetable.

Perhaps somebody could tell me if I'm missing something important, but aurally, the difference between the naive approach and this are night and day, an sound sufficiently DX7-y to me.

The other trick, if you're trying to design a matrix-style with abstractions, is to make sure that you keep the "original" operator output out of its input, and that the modulation index goes to your "clone". I've accomplished that with dollar variables, as above, (e.g. operator 1 mods operator 1, "[r op-$1-mod-$1]") and in my matrix abstraction:

Basic flow-control here, $1 is the modulator operator id and $2 is the carrier operator id. So if they're the same (feedback!) don't let the signal through.

Anyway, I hope this helps anybody. I'll get the full synth up when the interfaces are a bit more usable, I'm pleased with how simple it's turning out to be.