Azorlogh/harmoxen is an open source project licensed under Apache License 2.0 which is an OSI approved license.
Similar projects and alternatives to harmoxen based on common topics and language
Make xenharmonic music and create synthesizer tuning files for microtonal scales.
Music Composition System for S60 Symbian
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One music sequencer for all major platforms, desktop and mobile
norns library for defining microtonal scales and chords - play on grid as a isomorphic keyboard
Not enough MIDI notes!
reddit.com/r/isomorphickeyboards | 2021-05-03
Now for your first question, I don't know about doing it with arduino, but if you manage to find resources to send simple midi messages, it'll be the same thing to send MPE messages as well. As to what messages you should actually send however, I used this to understand how to send pitch bend range information, and learned the rest from the spec and a bit of tinkering. You can check out the source code of my custom piano roll which uses MPE to send notes with arbitrary frequencies. The relevant parts are the setup_mpe method, which just has to run once in the beginning to setup the synth to which you're connected. The other ones are note_on and note_off. (sidenote: actually here I'm not using the least recent channel because I was lazy when I wrote this, but)