OK update - so I pulled the top of the mast off and there was a bunch of old silicone in there. I was like "wtf how is this leaking?" but when I poked at it water came out. So I pulled it out and the foam behind it was saturated. So I pulled the FOAM out and there was a piece of black plastic there:





This appeared to be riveted in as well but I dunno what it was for so I just left it as-is. Water dripped out of the mast for about 15 minutes. When it stopped I dried it out with some paper towels and cleaned with some acetone.

When that was all dry I put a thick layer of silicone over the black plastic thing, then I inserted some sliced "foam pool noodle" pieces to fit tightly inside the mast, and put another thick layer of silicone over the foam. The result was this:




Then I put some silicone on the mast head and re-riveted it in place.

Tomorrow (after curing overnight) I'll carry it down to the water to make sure there are no bubbles. If all goes well I'll set the boat up and tip it a couple of times just to be sure.

Wish me luck!