If you're going to do a job that's worth doing, it's worth doing right (man, I sound like my father).

Seriously though - if you call Hobie, they will advise you against welding on the mast because of the liability. The heat treating process on aluminum is particular and the material properties will be changed for the worse by welding. If you MUST - you should really consider cutting the damage off and riveting a sleeved section back on.