So I don't think things are working as intended:
http://replay.pokemonshowdown.com/aqua-megamania-1792
0. No HP Percentage Mod, nor Team Preview, nor Sleep Clause, Moody Clause, Evasion Clause, Baton Pass Clause, whatever.
1. Mega Evolving immediately reverts me to Aria Forme.
2. I don't have Tough Claws despite setting that as my nickname. I think that's how it's supposed to work? Well, actually I might have Tough Claws that first time. I have no idea what's up with that calc, because:
252 Atk (107 base) Meloetta U-turn vs. 248 HP / 216+ Def Rotom-W: 48-57 (15.8 - 18.8%) -- possible 8HKO after Leftovers recovery
and
252 Atk (107 base) Tough Claws Meloetta U-turn vs. 248 HP / 216+ Def Rotom-W: 63-75 (20.7 - 24.7%) -- possible 5HKO after Leftovers recovery
My U-turn did 60 damage, which should not be possible for either of those.
I'm guessing it was a weird Rotom-W spread and that I did not, in fact, have Tough Claws.
Actually, there's one more possibility:
252 Atk (77 base) Tough Claws Meloetta U-turn vs. 248 HP / 216+ Def Rotom-W: 51-61 (16.8 - 20.1%) -- possible 7HKO after Leftovers recovery
This can deal 60 damage, but that means I never had my Mega stats at all.
3. Changing back to Pirouette Forme removes my Mega stats.