I have written a program which converts MIDI files into MUSIC lines which can be used in an IntyBASIC program. Depending on the number of voices in the MIDI file, it can generate code either for the Intellivision alone, or for the Intellivision with ECS. MIDI Program Change messages can be used to select among the four instruments that IntyBASIC supports. I have also attempted to support drums (on MIDI Channel 10).
The source code and README are available on GitHub:
Binaries for Mac OS X, Linux, and Windows are available on the "Releases" tab on GitHub, but I've also attached the binaries to this message.