Rain water coming in through the inside of the mast?
Yes, all keel stepped masts (especially /w internal halyards) allow water in.
One of the many reasons I hate shallow bilges w/out deep sump.
BTW: You likely suffer from siome mildew on the under side of your sole (floorboards); which can be treated /w chlorine bleach & scrubbing - then soak the underside of the boards /w ethylene-glycol type anti-freeze.
Interesting idea; converting keel stepped to deck stepped. I like the idea of being able to separate the mast (jettison at sea) from the hull.
Supspect, tho'', that the deck reinforcement at mast base could be problematic. Would''nt the lower mast stub only provide a compression post?
How about merely "sealing" the interior of the mast at the deck partner (or anywhere beneath the lowest mast opening), and drilling a weeping hole immediately above the seal?
I never bothered, as my diaphram bilge pump kept the deep sump pretty dry.
I''d like to hear more ...