No more electrolysis should happen to a bronze thru hull that is isolated from your grounds than if I just hung the thing in the water at the dock.
How can a difference in potential occur if the only thing touching the thru hull is the seawater it is immersed in.
Seems to me you need an anode and cathode.
Of course that is an interesting experiment... what happens to bronze if it is just hung in seawater without any other "connections?"