In case anyone's interested…..I've got this 80% figured out.
A MOSFET in series with original diodes handles the switching nicely when driven with a monostable square wave pulse at the gate. The signal from the piezo doesn't quite cut it directly, but feed it a square wave and it tracks the tempo nicely. Now I'm just figuring out how to use the signal from the piezo trigger in the simplest way possible so I can use the drum triggers to do tap tempo.
Should work with the external tap tempo for any pedals that use this three button switch arrangement.