drill a small hole in the mast cap toward the front (far away from the halyard sheave area) would be my first suggestion. If you intend to find your leak and fix it, patch the hole with some fiberglass. If you intend to leave it leaky (or want the possibility to drain it in the future) tap the hole and insert a nylon screw with o-ring with a little sealant.
I wouldn't put the hole in the top of the mast - it's subject to the most hydrostatic pressure when you turtle - and it's the only part of the mast in the water when you're capsized. Base of the mast sees more spray, but a lot less pressure when you flip.
