"claus" wrote
I am now leaning towards the theory that the prop is somehow electrically
isolated from the engine block (and the large zinc)-

That may well be the case, but it's not necessarily a bad thing. If you
haven't done so already, you might want to try to find out if it's that way
by design. Perhaps frequent relpacement of prop zincs is the price of
admission for your particular unit? There generally are no easy solutions to
galvanic corrosion, only trade-offs. For example, if you electrically
connect your prop to your drive unit, you might be encouraging its galvanic
currents to attack a nearby aluminum housing rather than a more distant
sacrificial anode.