I'm not sure of it not being affected at all...maybe less affected....2 dissimilar metals in salt water make a nice battery regardless. Stainless steel is more noble or cathodic than Aluminum for sure. Aluminum, which is anodic will be more affected by the copper (which is more towards the middle of the scale) for this reason, and the fact that the seperation on the nobility scale is greater between copper/aluminum than for copper/stainless...but I bet if you look at any of the paint manufacturers like Pettit or Interlux, both will not recommend using copper bottom paint directly on any metal parts in the water. They will tell you to barrier coat the parts with and epoxy primer like Interprotect and then use the antifouling paint on it, or one specifically designed for metal.
I would check with the manufacturer before applying any copper based paint onto any metal parts.
