Audio Units (AU ) are a system-level plug-in architecture provided by Core Audio in Apple’s macOS and iOS operating systems.
So, Audio Units is and Apple thing, that’s why you don’t see them.
It doesn’t appear as if Presonus have done any significant changes that would relevant for improving the MPE handling here. I don’t have the SO7, but I am looking at their description here, and it looks very much like the same architecture that was the problem in SO6.
Maybe there is an solution with the new ways of sending the MIDI OUT from MG3, but the safest bet is probably to set up MG3 in Standalone outside of SO7 and create a “new keyboard” (checking the MPE box) from the MIDI OUT you have there.