Bob: I had a stainless self-tapping screw trough an aluminium section on my rudderstocks. It failed last summer, with badly corroded alu where it went trough the stock (factory mounted).
All literature also advise against letting steel and alu be in contact without insulation between, even if you have good experience with it.

Gareth, the heads looks nice, but how do you plan to attach them to your beam? The sailcloth section of their site was not cheap..
Looking forward to hear about your experiences.