Well, trying to imitate the waveforms of the allophones is basically already a sort of proto-formant synthesis
Hybrid Qchan would be doable. Generally, almost any beeper engine can be combined with AY without too much trouble. The main problem is that adding AY would significantly increase row transition noise on most engines, because (re)loading the AY registers takes a lot of time. The other problem is volume balance. For example the classic Qchan is not a good candidate for combining with AY buzzer, because the buzzer is much, much louder. This is why I chose Squeeker-type synthesis for my hybrid experiment, because this type of synthesis produces very loud sound, and it also cloaks row transition clicks pretty well.
Well, as I said earlier, I've retired from writing beeper engines for the time being. So don't put your hopes on me. I do hope though that someone else will pick up these ideas, and I'm of course more than willing to help if somebody is to take up the challenge.