Gosh, you know what, I'd be calling the airline straight away and asking upfront.
We have so many couples coming onto our aircraft carrying big suit carriers of garments that need to be treated like gold leaf, and we simply do not have anywhere "special" to place such a "special" thing! There are coat cupboards in J class that are very strictly for J class only, and even if we were to be dodgey, they are only tall enough to fit coats, not dresses, so the bottoms get scrunched up anyway... It always ends up in a game of "see-how-many-hosties-can-get-into-an-overhead-locker" as two or three poor hosties in skirts play acrobatics in the overhead lockers trying to perfectly place the damn thing on top of peoples bags in already crammed overheads!
I've seen a dress get nastily damaged once - after the bride-to-be and hosties meticulously arranged this most amazingly beautiful dress, some late running douche runs on and shoves his huge buckley backpack on it and tore the dress! The bride to be was fine for some reason, it was me that was hysterically crying!
So ask, dont assume, and start conceiving a back-up plan for if your first idea doesn't work!