This might work if it was something that you could raise to the spreader level alongside a shroud (and you could store it at the base of the shroud). At that location it would still be accessible without diving (a few inches below the water) when the boat has turtled and being offset it would give off-balance buoyancy to help turn the mast. Being limited to spreader level cuts down on the lever arm though, but you could compensate by using a bigger float (even a large fender, perhaps).
Any "after-the-fact" system would be unable to prevent the boat from turtling initially, which may not be an issue in deep water, but could become tricky where the mast can get stuck in mud.
There is also the issue of being able to pull the float below the water with enough force to get it to dive. Can imagine that to be awkward.
Alternative idea: take a light (dyneema) line and fix it at the spot where the stays connect to the mast. Run the line down to the deck and tape it lightly to the stay (or use clips, anything that gives under strong enough pull). After capsize and turtling, you can unfasten the line at the deck and use it (perhaps from a boat) to pull the mast back up to water level.
So much for armchair speculation