The first approach currently seems more promising.
Maybe this whole thing is just a stupid idea…
Marking stereo pairs in the channel maps in the xml would be useful for automatic stereo downmixing.
Stereo downmix toggle button in the UI?
Drumkit file channel map has stereo channels marked with either left or right.
When active only the first two plugin output channels are used and marked stereo instrument channels are mapped to either output ch1 or ch2, the rest are mapped to both.