Peter, I'm not sure what, or where, a gz curve is: and it might be different for the 'big' Swallow boats, though I rather doubt it. I have a CF mast, 15ft long (for a 12 ft hull) and weighing about 2 1/2 kilos. It should have been sealed but wasn't (now definitely is!), and after 2 hours of inversion was pretty full of water. However the leaks were very small, and the inversion happened within about 2 minutes of the capsize. I don't believe a significant amount of theoretical mast buoyancy could possibly have been lost in that time. (Once the boat was upside down, I suppose the extra pressure 15 ft down might have squirted water a bit more quickly into the mast cavity?)
So, whatever the theory, I'm sceptical that a CF mast would prevent, or even delay by very much, the inversion of a super buoyant and therefore inversion-prone boat.