I used to be a flight attendant for USA 3000 (they've since gone out of business). We flew a lot of flights to Cancun and Punta Cana, so we had a lot of brides on our flights.
Our planes were single class configuration (no first class), so there wasn't a closet. Brides would call our helpdesk prior to the flight and they ALWAYS told them there was a closet. Don't ask me why. I don't think anyone from that department had ever actually stepped foot on one of our planes. Then the bride would get onboard and find out there wasn't one and be upset.
Going up to the gate agent and asking if you can board early is a great idea. You might also ask how full the flight is. If it's not completely full, I would hold onto your dress at your seat until boarding is complete, then you'll be able to find an empty bin to put it in.
The hard truth:
--Yes, you may have to put it on top of some people's suitcases. Lay it flat and have your (hopefully tall**) fiance place it up there.
--Yes, the best spot for it might be all the way in the back of the plane--no where near your seat--but if that's where the empty bins are, then that's what you get. As a planning bride now--I get it that it's your most prized possession at that moment. But no one is going to touch it in flight, believe me. The bummer is that when you arrive, you may have to wait to get off last to retrieve it. Don't worry, your transport won't leave without you.
--You have to be flexible and mentally prepare for the worst--it gets crammed into a bin. It may get wrinkled, so budget for steaming. But at least you will ensure that it gets down there!
I bought my dress about a month ago and the train is much larger than I ever anticipated. It's heavy and it's big so I will be having the same problem as you! My plan is to give my fiance the dress as we board the plane. (Flight attendants always deal better with men than scatterbrained/protective women--don't tell them I said that!!). He will be instructed to place it nicely on top of some luggage and then close the bin doors. Then he'll have to guard it. Under no circumstances will he let a flight attendant or a passenger cram stuff on top of it!
And when all else fails--BRING CANDY! (just make sure it's sealed in original packaging--no one trusts open stuff in this post-9/11 era) Bribery works well in airports and on airplanes. You want the gate agent to let you on before everyone else?? Swedish fish. You want the flight attendants to save you bin space? Godiva. Give it to the first F/a you see as you get on the plane and tell them your situation. Never hurts!
Good luck everybody!!!