GREAT idea with the expanding foam in the mast... why have I never heard of it before? Think about it: doesn't add any buoyancy, but does the job of keeping water out of mast, without the added wind drag of a float-bob.
The foam is heavier than air, so you might want to check by how much... ;')
Anyway, I don't think that foam (e.g. 'Great Stuff') is corrosive, as it's used in contact with all kinds of metals, incl. aluminum.
Don't know what to tell you about the bolt-rope tracks, but you seem to be very crafty at keeping the old gal lakeworthy!
The expandable foam will absorb water over time (if exposed) and will become very heavy and impossible to remove. It also does take away from the buoyancy by adding material inside the mast (air is pretty much as bouyant as you can get). Remove the existing hardware and reinstall it with 3M 5200 sealant or the like and/or plug up the holes with it. You'll be sorry about the foam in a couple of years.
