If the zinc wasn't necessary on the Onan it could easily have been replaced with a standard pipe plug, and heat exchangers are easy and (relatively) cheap to have custom made without a zinc fitting. If those were good ideas, Amel would have done one or the other.
That zinc is needed to protect the materials in the Onan heat exchanger. It is NOT redundant with the rudder zinc. If the rudder zinc protected the Onan heat exchanger, why bother to replace the zinc when it dissolved?
Rose Island, Bahamas