I personally wouldn't be concerned about a splice in mast, especially if it is close to either end. I think everyone is blowing the weight issue all out of proportion. lets say you used a sleeve 4" in diameter x 1/4" thick and 2 feet long. You are talking about 6 lbs addition weight! That really isn't anything to sweat. If you mast isn't round you are probably going to need to have a couple of flat plates press brake bent into curved sections that match the inside of the mast. You might need 3 or 4 pieces and I wouldn't bother welding them together. Just rivet or bolt them inside the mast.
But what if breaks? Well it was broken before, so now I'm going to have to replace it!
Gary H. Lucas