The Commodordion

I've kind of done this, just without the actual "build" part—I instead use a Wooting computer keyboard (which has the hall effect switches you're referring to), and I've written some program to emit MIDI based on key velocity. I've been using it very frequently for a few years, and can confirm it's quite fun to use, though I don't have much experience playing other keyboard instruments and haven't tried to learn it seriously enough to make proper use of chords etc. Demo of me playing here, though try not to mind the messy desk and bad recording setup (one-hand, MIDI going through phone due to lack of speakers): [link] The layout I use is not a regular piano layout, but a B-griff layout, as used on a bayan (Russian accordion). I made a demo site for playing with the layout (keyboard or touchscreen), but without velocity sensitivity: [link] The B-griff/C-griff layouts naturally fit computer keyboards, and I'd argue they're better in general than piano layouts for various reasons. There's also this cool video of someone using two Commodores for similar effect: [this video] EDIT: turns out the Terpstra keyboard is also able to work using B-griff: [link]