XR OSC Midi only transmits across the usb interface or physical midi ports so Mixstation has no way to get there
You can however have Xair application on a host pc plugged into the xr and it can then communicate to GP
To my knowledge, you cannot get across to the xr without usb directly (or networked midi which then needs hardwired connection to the xr)
Mixstation only addresses functions on the xr because its fixed schema if you know what I mean, so Im not sure what faders you are thinking of using?
Also, is this for the FOH guy because it would be tricky using mixstation during performance to tweak the plugins for a player?
Not sure if this helps but I had a pretty difficult time putting the pieces but check the explanation and tell me if the diagram helps (xr mixers are a 'fixed schema host…they already have everything mapped and way beyond what is available in midi)