I have done several 20-note arrangements for a friend who owns both
a German 20er organ playing from an SD card and an accordion (I think
from a different maker) which accepts the same cards and synchronizes
via a wireless connection.

I had no particular problem with the 20-note tunes -- you just
transpose to a G-based scale (not the Raffin F-based scale) and save
in a Type 0 MIDI file.  I know the overall file structure is capable
of much more.  Original files include percussion and lyrics and some
files play multi-instrument arrangements -- i.e., it is capable of
true duets, not just multiple instruments playing the same part.

Does anyone know where I can get a more complete description of this
MIDI file "scale" (or whatever you call a format driving instruments
built to different "tracker" scales)?

Wallace Venable