I made some changes to PPMCK, including tail recursion, custom tuning, labels, and command to include your own effect codes in a music. I make some musics with it.
The custom tuning idea sounds pretty neat, I've been curious about trying something with that sometime. If you have some NSFs/MMLs to post sometime that use it, that would be cool to see. If you need a place to upload, I can give you an account on my server.
I have not posted the program yet, but I have made three files (one of which uses custom tuning).
Here they are (not much yet, sorry):
http://zzo38computer.org/prog/mck/music/
Some more info on what new features you've added and how they're supposed to be used would be good, if other people want to try them.