Some planes have closets but not all, so I wouldn't count on it.
Carnival Dream for Thanksgiving
Eastern Caribbean- 11/20/2010
Carnival Ecstasy (4 Day Baja Mexico) 2003
Carnival Pride (7 Day Mexican Riviera) 2007
Carnival Spirit (8 Day Exotic Mexican Riviera) 2007
Carnival Freedom (7 Day Western Caribbean) 2008
Carnival Elation (4 Day Baja Mexico) 2009
Carnival Spirit (7 Day Southbound Alaska) 2009
your best bet would be to have your ta call the airline for you. If you didn't use a ta then i would call the airline, not reservations but customer service tell them exactly what you told us, give them your flight number and hope for the best, otherwise you are going to have to pack it in a garment bag and check it in
another option that you might consider is that if you are staying at a hotel pre-cruise have your dress shipped to the hotel, have UPS or FedEX or whatever box it up for you, ship it insured to the hotel, call the hotel and tell them what you are doing.