Hopefully abercroft will be able to help. It would be hard to not only hire a dress like you described, but even hired gowns are usually fitted in some way, aren't they? So that would still require some time, most likely, not to mention all the time involved with even
looking for a new dress and finding one you like!
I was going to suggest what Bookgrl said, you could put a panel in the back - one that laces up, which wouldn't look odd, especially on the style of dress you described. Plus you could add some lace or something to the bottom, couldn't you? Hopefully it will work out some how, some way. Fingers crossed for you! What a nightmare!