David: thanks for the link to the standpipe project (by the way, I got completely distracted by the rudder rebuild project: I had no idea that foam could be that powerful...). My mast is stepped on a tabernacle with a foot (including a heel) at the mast bottom. This is common on the E25 CB, so I'll put two weep holes just above the foot where it fits the mast bottom.
The original mast wiring had slabs of 2" cushion foam spaced every 3 feet securing it inside the mast, so the top few absorbed most of the rain and were quite black and disintegrated. Very little moisture seems to have reached the mast bottom over the years. I'm now using heavy duty ties in a star configuration every 3 feet around the mast wiring, and I was concerned that moisture would now pool in the bottom.
From the replies I'm not going to bother further with the issue. If the outside of the mast gets wet, the inside will just have to join in
.