Wait, don't give up yet!!
In Richard Henderson's book, "Understanding Rigs and Rigging," he shows a diagram of something similar:
1. a balloon to fill the mast and block it off
2. poured closed-cell foam on top of the balloon to level it off
3. poured epoxy over the top of the foam.
The foam and epoxy are done with the boat healed so that the top surface is sloped to promote drainage, and this is done at the level of the lowest halyard exit, so no need to drill extra holes. Make sure that the conduit for you wiring extends well above the block.
I have not done this (yet) myself, so I can't comment on how well it works.