Why don't you just seal the mast? The mast is wide open at the bottom, but you have to worry about the halyard that runs through a hole in the side of the mast. There is already a small plastic tube through the foam plug at the top of the mast, so if you extend that all the way down to the exit hole on the side of the mast, then you can seal off the inside of the mast above that hole. You'll have 30 feet of empty mast for volume.

Granted, it will be a pain in the butt because you'll have to take apart all the downhaul stuff inside the mast, but it beats trying to right a turtled boat with a mast full of water!