Up you go! Just wanted to post the same feature request myself ... and I am actually surprised it is not that requested. Not sure of people are not using the Mix FX that much, or side chaining that much, or simply have not noticed.
It's quite annoying, especially since one of the reason I got Studio One was for the Mix FX engine. But this (arguably) bug makes the Mix FX basically unusable for me, as I use a lot of side chaining, from regular volume ducking, to spectral ducking (track spacer like), vocoding or similar effect also use side chaining... Ryan's workaround is not really feasible when you need to create duplicates for tens of tracks, it becomes a huge mess.
I don't know, to me it should be pretty easy to detect if the side chained signal is used for detection only, or it actually passes trough and is heard. Or at the worst, a setting for the side chain could be added (similar you how you can say if it should be pre or post fader), where you can specify that manually, which would then tell to the Mix FX plugin if it should affect the target channel or not (I would assume most of the time you would want it to be affected, i never had, nor can I now think of a situation in which I would not want to).