If having 8-voice and constantly play 4-voice polyphony is problably not acceptable workaround.
I might look at making an own external flip switch that decides if REV2 detect plugs in B outputs.
It's usually very simple physical connector that makes contact when plug is inserted. Very common and often used on phones output too, if to silent onboard speakers and such.
So have a 2-pole flip switch on back of REV2, where each pole is in series with the switch on the femail connector. Should not be a problem to find a spot to put it, I think.
So if anything plugged into B outputs, that switch tells synth if to sum output to A output or not.
An even more sophisticated approach could be to pick signal to buttons Stack or Split, and when lit draws a relay that do the switch for you. This relay is in series also as mentioned above.
- then assuming stacked or split always use B outs if plugged in
So all automatic in relay case.
Have fun....