You are probably going to want to clock the MoXS with the PSR, then (otherwise your registrations' tempo controls won't work).

I'd take a serious dig into the XS's MIDI implementation documents to see how to turn off the reception of everything BUT the clock signal.

You might want to post on the XS Motifator forum. I'm sure someone has done a similar thing (even if not with an arranger) over there...