Mike, as far as I know, Loopcart was never finished. It works however, just lacks some functions but you can adjust and make beats. There's also Slocum's Atari 2600 Sequencer, which is mostly editing text file, writing Song/Pattern/Beat data in binary/hex/decimal format. Not difficult, documented well, but not real time sequencing, just for writing songs to format that can be compiled by dasm.
I don't know anything else, I have written some software to my own purposes but so far they all sounds rather primitive (in bad way)