I can finally go in the dressing room but I am still trying to figure out what you could do in there. I think there has to be some sort of award for beating pokestar studios. But what is it?
From what I gathered wandering around there, it's just a dressing room and you can't do anything special in that room.
Serebii doesn't say anything about it, and I couldn't find anything special after exploring all of it.