Since making the new bellows and fiddling with the pressure-to-Expression transfer function, I switched my attention to making a dedicated BLE-driven synthesizer I can use with the concertina.
That turned out to be pretty straightforward. I based it on a Raspberry Pi B+, a touch-screen display from Adafruit and a MAX9744 Class D Audio Amplifier, also from Adafruit, Taking inspiration from the Adafruit Pibow stacked acrylic enclosure, I designed and laser-cut a custom enclosure for the hardware. This worked and looks nice, but, because of the need to interconnect boards, is a bit fiddly to assemble.
For software I used Raspbian together with a selection standard packages. For the synth, I chose the open source Fluidsynth project. On this set of giants' shoulders, I wrote a simple GUI client for Fluidsynth using C++ and QT5.
Et voilà! A fully functional synth in a nice compact form factor with just the features I need to have exposed.
Since getting the synth working, I've stopped fiddling with the innards of the concertina and the synth and just used them. I play the concertina almost every day and it's a joy to do so. Mostly, I make it sound like a flute, an oboe, a bandoneon, or the pan pipes. But a cello and a reed organ are nice, too. Maybe I should turn my attention to making an English concertina "sound font" so that it can sound like what it appears to be. Maybe someday. In the meantime, I think I'll just use it.