IANAL but... isn't this somewhat analogous to side-loading apps on iOS or Android? As in, Google/Apple make it awkward for you to provide your own files to run on your phone, but ultimately it's not impossible nor illegal.

Likewise, if you're able to upload your own custom MIDI (or other) files for your piano to play, then all you'd be doing is sharing a utility that creates mixed audio/MIDI MP3s, which may or may not be used to create files which can then be legitimately uploaded to a piano.