[hamiam]

Drill it, tap it, and install the cleat. There are various products on the market that can be used to separate the metals. I think that the Mike above is far too worried; the wall of your mast, unless you have other problems, is quite strong and, if the roller furling system is anything like mine, the load on the cleat with not be extreme. The alternative of putting a fitting in the deck is fine if you spend the time to dig out a radius of core around the hole and then seal the whole thing with adhesive.