Probably a dumb question, but why do you want to cover the mast? To protect the mast? To prevent water from getting into the existing sail cover? Or....?
I would think trying to put a curved zipper in would be daunting. If you sew the outer zipper tape first, the inner one is going to have to be gathered because it will have ripples in it (or you would have to gather the canvas material to fit the inner zipper tape, which would also be difficult because of the thickness of canvas). I would think it would be easier to design a cover that can take a normal, straight zipper application.