Some people have considered stuffing pool noodles inside the mast. I don't think it would work all that well, what with rivets protruding inside from spreaders and tangs and such… Plus, any weight aloft is heeling weight.
I think Centerline has it right, just tie a jug to the top of the mast. Ugly, but effective. A guy at the club did that with his Flying Scot last summer for our capsize clinic. As long as the Flying Scot mast doesn't go under, a Scot will float on its side, and not take on any water until you right it. The Scot does have a class legal masthead float, which is a pita-like pocket of foam that fits over the head of the sail. The racers call it "the diaper," as in "It's really windy today, I think I put on the diaper."
It's a great idea to get your boat out with a support boat standing by, perhaps in chest deep water, and deliberately capsize it a few times, to see how it will behave. You can practice climbing over the high side right out onto the centerboard. Or see how long she might lay on her side before turtling. I find my 15 footer will lay on her side if there's no wind, but if there's any wind pushing on the hull, she'll turtle. She has a foam filled mast (from the manufacturer) and was supposed to have a mast head float sewn into the head of the main, but I had a new main made without the float.
From my years of learning on a Sunfish, when the boat goes over (I haven't had her go over on me yet, just on purpose when I intended) it's almost instinctive for me to climb over the high side onto the centerboard. Last time that happened with my 14 footer, I went onto the board, got her upright and climbed back into the boat as she righted, and didn't even get my bathing suit wet. Of course, that boat has side decks, and she ships a lot of water, so I got to bail for 45 minutes. A 1 gallon jug used as a bailer can't hold a candle to a bucket when we're dealing with 10" or 12" of water in the bottom!!!