Finding old software for it can be difficult, but programming books are still floating out there. Enter "C Programming for MIDI". It details an Alpha Juno patch editor/librarian that uses an MPU-401 interface.
I've tried to find the original files or a copy of the floppy, but they're nowhere to be found. So, I had to copy the program out of the book.
The book has a number of mistakes: typos, wrong variable names, missing values/functions, etc. I fixed enough typos, and added few enough, that it compiles and seems to run correctly.
The source and binary are on github here.
It's almost useless today and was fairly basic in its day, but I think it's a decent starting point for making a DOS MIDI program.