Michael, that would be a great way to go: the STM32 line is very powerful. You could look into the Teensy++ MCU boards as well. The latter is already being used for a new Prophet 600 control core. Chip, alias Synth Hacker has used an Arduino Mega board to replace the Korg Polysix Key Assigner.
I can code in C, so if time allows, I could contribute.
Yash