Ron,
Minimum shear strength of one 3/16", aluminum rivet is 380 lbs X 2 ~= 760 lbs. Working load of the Harken 2156 is 300 lbs. Breaking load is 950 lbs. (With a 4:1 system, I sure hope you never break the 2156).
Whenever I match stainless with aluminum, I use Corrosion X for threads, and I use electrical tape as a gasket between a piece of hardware and its mounting surface. I just stick it on and trim the edges with an exacto.
I wonder if putting a small section of shrink wrap around a rivet before putting it in the hole would provide the separation needed to prevent electrolysis? Yeah it's going to get squashed a bit maybe, but it might just work.