An envelope follower that can be mapped to any parameter. maybe it would work best with a sample&hold mode that updates every time a new peak is detected.
Maybe some sort of parallel processing, so that the last excitation is not affected when changing a parameter?
Thanks for your valuable input! We have been experimenting with bitwigs built-in capabilities to derive envelopes from the audio input and used these to map them onto the controls of the plugin with great results.
The parallel processing idea is conceptually a little bit difficult. Would it be a polyphonic instrument then? When do we know if a new hit starts when playing with live audio input? We would need to deal with some uncertainty and possibly latency when we need to spectrally process the audio input to detect attacks.
It is easier to achieve true polyphony with multiple piezo discs attached to multiple resonators. Then changing the parameter of one resonator wouldn’t affect the other.
E.g. you could load the plugin multiple times.
Cool. I am using ableton lite, so I don’t have access to M4L and there does not seem to be a straight forward envelope to midi CC plugin out there (for Mac, at least)
I can see how the parallel processing thing would be challenging to implement properly and probably not worth the effort.
On the other hand, I’m thinking about ways to use a single piezo for multiple instances of cymbal/snare, because I only have two channels in my mixer and one is used for guitar. I‘m thinking differently pitched tubular drums, running into band pass filters and then layered with white noise… no idea if the crosstalk would be reasonable or if too much of the subtleties would get lost in the process. Most likely, yes… 
have you tried something like this?
Cheers!