I had a centerboard break at the hook too (actually, I bought the boat that way knowing I would have to repair it). The problem is that the fiberglass reinforcement doesn't extend all the way around to the end of the hook. Most of the tip of the hook is just gelcoat, so it doesn't have adequate structrual reinforcement.
If I recall correctly, I fixed mine by making a rough mold out of masking tape around the tip of the centerboard. Then poured in polyester resin and let it cure. Then filed it to the correct shape and laminated a couple layers of fiberglass cloth around the tip. Once you get some glass around the tip, it should be plenty strong.