Susan Marie, I had quite a long look at this last night, but I'm yet to reach any firm conclusions. The harmonisers that I've found to date tend to provide a harmony backing based around chords and derivations of chords (arpeggios). I've downloaded Sweet Midi Harmoniser but haven't got to first base yet. And I've posed your question at the Music Siftware Discussion Forum at Software Music Machine.

The thought also strikes me that the basic text nature of ABC may lend itself to writing simple thirds or fifths - mabe Bruce O could comment on this.

Likewise the numeric structure of the Miditext files generated by ALan's Mudcat program may also have some possibilities for simple data manipulation, but at this point I don't know how to interpret the numbers.

Just a few thoughts. Regards, John