As far as sealing the inside of the spar, Selden does a similar thing to the spartite. They make a dam and pour something inside the mast above the partners and it prevents water from making it below. It works well for that purpose, but what a pain if you need to run something else up the spar. I can't remember what it was but we once needed to install one more wire in the spar of a new boat that wouldn't fit in the conduit. Some people add a lot of stuff. I made a 12' hole saw out of a piece of aluminum pipe and was able to get through it but there was no way to really seal it again.
In my opinion, water coming down the inside of a mast shouldn't be a problem. It's fresh water, so as long as there are clean, adequate sized limber holes, the step isn't sitting in a puddle of bilge water and there is a little ventilation, not much is going to happen to it.
I usually apply a coat of grease to the spar where it contacts the step or shoe to help ward off corrosion.