Spray foam is the last thing I would shoot into a mast! Anybody who has every used it doesn't have to ask why. Lets just say it would be a permanent solution, then you will be pouring solvents down the mast trying to dissove it next time you have to service
something or pull new wire.
Maybe if you ran a couple conduits for wiring
and halyards then shot some foam, but even then I think it's a bad idea.
Home owners love that stuff, think it's the miracle solution, then they call us contractors to deal with the results! Try pulling a wire that is even partially encased in foam, good freakin' luck! Permanent solution!