All of BASSMIDI's processing is currently limited to stereo, so it wouldn't be a simple addition, but not impossible either. If it was to be implemented, it would probably be by producing multiple stereo outputs that get combined at the end for multi-channel output, but custom processing via BASS_MIDI_StreamGetChannel would probably be impossible then (as there would be multiple streams for each channel/effect). I'll look into it before the next BASSMIDI release, but that could be a little while from now, as the last release only just happened.
In the meantime, you could indeed implement it by creating multiple streams and muting different MIDI channels (via MIDI_EVENT_MIXLEVEL) in each. If you link the streams via BASS_ChannelSetLink then they should start playing in perfect sync. Or you could use a BASSmix mixer to combine them, or even a simple STREAMPROC (that fetches and combines the data from both).