Well, you're probably looking at it the wrong way; if something
mediation
committee-like is being talked about, telling X or Z what to do is not
the
purpose. Instead it's to get X and Z to (grudgingly) meet at Y. I agree that the general scope of this needs to be clarified, however. If it's being talked about in the same vein as the global arbitration committee, there are problems (the same problems, in fact).
This is fine with me, but do X and Z work on an article in the same project (say Swahili Wikibooks)? If yes, then this project has its own conflict resolution avenues (if it does not we are back to the Mediation Committee issue) which they soon or later will be obliged to follow. If they do not work on an article I am afraid they can argue with each other till eternity on Meta and no committee would ever help.
Cheers Yaroslav