Hi @robertcthomas !
The idea you suggested can be done with littlebits and some electronics at the side !
You need for each piezo an op amp amplifier, followed by a opamp threshold so the output will be a clean 5V pulse.
Then, connect all these outputs to a resistor divider network, so you'll end up with an analogue voltage, ranging from 0V to 5V, depending on the piezo triggered. Feed this voltage into the littlebits oscillator synth, combined with other bits to create the sound you need.
Et voilà, there you got your piezo synth !
And don't forget to publish this invention !