There is a liquid rubber/eurythane material available that I researched when I was considering making my own CatRax wings for my Prindle 18.2. I was considering using it as a crossbar insert to support and cushion the wing inserts. It's a liquid that when it cures, solidifies at varying hardness levels, from soft rubber up to as hard as a bowling ball. If you first insert a shaped plug into the mast and coat the inside of the mast with mold-release, you could pour the liquid into the upright mast and let it cure and harden. Then pull out the hardened insert, pull up the plug and you have a perfect form-fitted mast insert from which to slide on the other half of broken mast. I don't think there's a way to actually get the insert out once you repair the mast, so if you epoxied it place, you would just have additional weight on your mast at that point of the break. But it would also provide an additional measure of structural rigidity at the point of the break.

What kind of mast? Carbon? Aluminum? How would you repair it? Weld it? Glue it? Welding aluminum is not the easiest thing. And to be honest, I believe the mast would be considerably structurally weaker than before that it wouldn't last long and fail again. This may be a futile endevour. Masts and rigging undergo signifigant stresses under sail.

I don't have the URL to the company that makes the stuff with me right now. If you PM or e-mail me, I can find it when I go home tonight.