I think I have two possible solution to running wires thru the cabin to the mast:
Solution #1: Use a deck-mounted plug: Waterproof Plug Connector
This method has the advantage of being easy to disconnect. It has the disadvantage that the wires coming out of the mast look like they need to be in a single jacket. Since wire runs are going to two different locations (steaming & anchor), it will be a pain getting them into a single plug.
I'm also not sure how to deal with those little screws - I can screw them right thru to the underlying wood, but then there's the potential for rot near the mast step - or I can drill out and fill with epoxy, and screw right into that, but that's not much holding power. Of course there shouldn't be much stress on the screws.
I also don't know what doing the connections inside the plug entails. I prefer crimping to soldering but that may not be an option.
Solution #2: mount a watertight junction box on the mast, and run wires in a conduit (white hose) from a thru-hull in cabin top into junction box, and connect wires to mast wires at a buss bar.
Not sure what the advantage here is over the deck-mounted plug above. It'd be easy to run more wires I suppose without needing an additional plug.
So I'm leaning towards #1...