The Epeios MIDI to XML converter makes it possible to convert binary MIDI files to a format that allows them to be handled effectively with tools like CVS. If you put, using your favorite MIDI sequencer, the CVS '$Id$' tag (or other CVS tags) in the comment tag of the MIDI file, the changes made to this tag by CVS are propagated into the MIDI file.
Scheme Note is a Scheme-like music file format and MIDI conversion tool. It takes a text file, which is a representation of a song, loosely tied to a MIDI-like event structure, and outputs a corresponding MIDI file. It is hand-editable for simple songs, and should be very easy to parse for a GUI.