George,
I'm no XP50 expert but I do have one and don't play it much anymore - sad to say.
To the best of my knowledge, the answer to your question is no - sort of, but not directly.
The XP50 DOES support looping, using RPS, but not in the style sense. You can load midi files that assign (RPS) phrases to specific keys. These can be patterns, fills variations and so on, but I'm not sure about intros/endings. You are basically playing a series of midi loops, patterns triggered by specific keys. The only way to stop the ones I've tinkered with is to stop MIDI.
I don't know of any specific [internet] resource for this RPS stuff - other than the roland demos. Some of the files sections in the JV/XP ring had some custom made at one time. Trouble is the XP80, 60 and 30 followed to quickly after. You may find some on sites that have the XP series - as RPS is common to all XP synths.
Hope this was somewhat useful..
regards,
Steve