Most of your problem with the PSR's is that MIDI tends to be an 'all or nothing' affair, from what I have read. In other words, you can't turn individual Parts on and off, the PSR sends them all (please someone correct me if I'm wrong). So you have to be fairly vigilant at the MoXS end to make sure that it DOESN'T play those Parts as well as just receiving clock.

Are you planning to clock the XS from the PSR, or the other way round?