The 733 has a smaller half height half depth closet that won't even come close to hanging a wedding dress straight. Most FA's will allow you to hang it, but FC coats will take priority. You can either take your chances with it half hanging/half crumpling with bags possibly being set on it or place it flat in the OH bin and keep an eye on it during boarding so people don't set things on it.

Maybe if you can find a bag that folds in half and hangs from both ends it would fit better in the closet.