If the tip weight rule is nixed then people with money will go carbon and whether you can utilize the unique aspects of carbon to make your boat faster is almost besides the point. Would anyone buy a new A cat these days with an aluminum mast?
From a basic point of view - Getting a carbon mast is something like a 3000USD option... maybe it is different elsewhere and I sure you could find an older mast that would work BUT I think it would outprice a not so insignificant percentage of possible buyers.
BTW... I love the carbon mast on the Jav2 18HT. It is one reason why the boat works so well and I also think it is more durable than any other mast I have seen.