The BX Stereomaker looks interesting but after looking at the manual from what I can see you cannot insert any effects into either the M or S components. They seem be to be creating the MS components internally
(by analog filters using digital science!) and then decoding back to stereo. The only control you seem to have is over what they allow you to adjust in between encoding and decoding back.
Also the OP mentioned he was not interested in stereo widening devices and this looks like it is more in that vein. Could be interesting and worth a try if you can demo it for example. Where that could work well is if you had an older (hardware) synth that is polyphonic for example an Oberheim but only has a mono output. That plugin sounds like it could widen it out nicely. But these days most virtual instruments have great stereo images already so you don't have to bother with that sort of thing too much.