It is my experience that FA's go out of their way to help you to store special and odd objects, and I expect that a wedding dress will fit in that category. For a while, Mrs. Arf was in the habit of buying rather larger and exotic bouquets of flowers to bring home (this is on domestic flights). The first time that happened I warned here that they would get squished, etc., but as soon as the FA saw the flowers she said that she would make sure that they were kept safely, and indeed they were. This was the case in subsequent times as well. The excitement of having the dress on board will make the FA's take extra care, I am sure of it.