I can confirm that Meloetta's Relic Song's sleep is blocked if it would break the Sleep Clause on PS, as I use mix Meloetta most often (which means regular forme switching) however, the main issue is with using her in wifi battles. If you're using a physical Meloetta and you sleep a Pokemon, you can't use her for the rest of them game (if you switch out, and the other poke doesn't wake) for fear of instantly losing the game. Mix Meloetta also has that problem but at least she can be locked into her normal form and remain viable.
At the moment I'm unsure whether she should remain in OU or drop to UU, because she is actually a pretty good bulky special attacker with only two weaknesses and, assuming you get the one turn set-up required (which is just a matter of predicting right, really) Meloetta-P can really do work with dual STAB and Shadow Claw or U-Turn, and it outspeed Tornadus-T with 200 EVs and a positive nature.
I guess the problem with Meloetta is that its job can usually be done by something better. The only thing nothing else can do is spam Relic Song, forcing switches. But the problem with that is, if you put something to sleep (in wifi) then Meloetta is stuck in whatever forme you left her in until you switch out, unless you want to risk losing the game (in wifi).
My main worry with Meloetta is that, because of the way Sleep Clause works on Showdown, it will get more use there than on wifi simply because it's more viable on Showdown than on the original game, and that that will effectively put it in the wrong tier...
But anyway, this isn't really the place to be discussing Sleep Clause, or what it means for Meloetta. I would just like to see her in UU xD
EDIT: Although saying that, one of the Pokes I have taken to using with Meloetta is Chandelure, which is likely to rise to OU when Shadow Tag gets released :L