I think your initial idea was the best. If you seal all the openings where hardware is attached to the mast, it won''t prevent the boat from turning turtle, but it will slow it down greatly, and give you more time to either right the boat or to attach flotation to the masthead.
When you see that the boat is about to capsize, step on the centerboard as it comes out of the water, and if you are quick, you can prevent the boat from going over, without even getting your feet wet. I thought it sounded improbable when a friend suggested it to me, but I tried it and it is not as difficult as it sounds.
As for re-boarding the boat after a capsize, my suggestion is that the physically stronger person should help the weaker to get aboard, and then the other can reciprocate.
Capsizing is one of the incentives sailors have to move up to a bigger, ballasted boat.