There is no need for an external anode to be attached to the engine block as there is nothing in the engine that needs protection - that is why you observe no wear. Connecting it to the saildrive housing may reduce wear of the ring anode. However, the effect is likely to be minimal as the ring anode wears first because of its proximity to the aluminium casing. That is exactly why it is attached directly to the casing. The best you can expect from an external anode is that it might be effective if the ring disappears completely, but even then it depends on how physically close it is to the casing.