I remember seeing some past discussions of quarter tones in which an unofficial abc extension was mentioned. Does anyone have the details?

To try out quarter tones within HARMONY, you could create a style file called, perhaps, UPaQtr.HST with the following two lines

1,130,10192
99,130,8192

If you apply this style file to a note (from the edit menu), the first line will shift the pitch up by approx. a quarter tone, and the second will unset this at the end of the note.

To fine tune the pitch, adjust the amount 10192 up or down (8192 reverts to standard pitch, 12192 is about a semitone above)

I could apply this automatically if the abc file included quarter tone indicators.