Unpurchased FC seats are usually given to the frequent fliers with platinum or gold status. Quite often, these people are upgraded 24 hours before the flight, so you really should call your airline, or have your TA call if you're booking through a TA. If you're willing to pay for the upgrade, go ahead and call now to ask about the upgrade. I'm a flight attendant, and we aren't allowed to upgrade anyone, even if the plane is leaving with empty FC seats. Only the ticketing/gate agents can do upgrades. I don't know about other airlines, but usually it's just annoying when people get all the way on to the plane and ask me for an upgrade.
FC closets are usually very small, so an overhead bin is usually the best place to put your dress.