Hello!
For using of multychannel instruments by rewire without IAC driver (on OSX), there is needed more, then just midi channel in Expression Maps. It must be as in out routing on track in arrangement - with possibility to route out signal to midi OR REWIRE channels.
Best regards!
Vadim
Yes, I see where you are going with this 
… it would be nice, but I think this is the same difficulty as asking VST Expression to be able to address multiple “regular” MIDI ports from the same track… (I suppose it would require something like the Drum Map)
It may be “VST Expression routing” tab in VST connection window )