ZynAddSubFX as a standalone programm is very exxective way to make music, since it can be comunicated via 16 midi channels to 16 parts. this is an awesomw feature. It allows you just to save the whole set of parts in zyn and a project within the sequensor of choise.
But the inability to record in multichannel mode (each part to it's pair of channels at least) makes recording 5 minute piece with 10 parts a 50 minute job.
So output channels matrix would be a saver feature.
Are you familiar with the jack-multi output option? This exports each part as a separate jack left/right port pair.
Wow that's really nice! Although before I could not find about it anywhere (was googling "zynaddsubfx multichannel output").
I see there is one difference with "-O jack" that "-O jack-multi" requires zyn buffer size to be same as jackd's one. But nevertheless multichannel recording is possible!
Multi channel output is also possible with oss-multi backend and virtual_oss (FreeBSD).
I see this feature is already supported in newer versions of ZynAddSubfx. Closing this feature request for now.