Update: I was still getting occasional triggers even after grounding the bow inputs - I just think there's something wrong with my board. I had to accept that, problems aside, shoehorning a Megadrum board into this application was a waste of its resources while it also limited what I could do. I wanted to be able to send CC messages with button presses, display custom text on the screen with each press (preset names) and associate an LED to each switch, so I used an Arduino. I'd never written a line of code in my life but somehow managed to get the simple functions I want to work, though the code I wrote would probably make an experienced coder's eyes bleed
. The interesting part is that the cheap clone I'm using can't do MIDI over USB, so I made a little MIDI-USB interface using that part of the circuit in the Megadrum schematics. Connected it to the Tx pin of the Arduino and it works great! So in a way, Megadrum remains part of the project.
If it ain't broken... fix it until it is.