Bill:
I sure am no expert in electrolysis, and don't have as much of a problem here that you salt water sailors do. But I don't know why you would want a sacrificial plate on the upper and lower ends of the rudder. Not being connected to anything/electrically isolated from everything, would there would be any voltage differential on the shoe that would create electrolysis?
In fact, having dissimilar metals (the lower rudder shaft and the zinc plate) are you not creating electrolysis where there was none to start with? I understand that the shafts are a mixture of metals, but still question whether there is an electrolysis issue.
The zinc plate for the upper rudder shaft might create the same problem?