Thanks for info on Yanmar

I've done some more research on the anode subject.

If you follow the link, I recommend saving the Western Mariner PDF and opening it in a PDF reader. The key articles are pages 11-17.
The Mil-Spec aluminum anodes are not just any lump of aluminum. There is compelling evidence they are better then zinc in several ways.
It is no coincidence that Crowley Maritime protects their vessels exclusively with aluminum anodes and gets several years out of them between drydockings.
One of the advantages of the aluminum ones is that they self-clean better than zinc (alloy) anodes.

The Navy did a ton of research to develop the aluminum alloy anodes that they use throughout their fleet. The small amount of zinc and the trace of Indium in the alloy help assure that the anode does not develop an insulating coating and that it will keep working as it gets depleted. The MilSpec really means something.

For the same level of protection:
Zinc Anodes: 24.5 lbs/ampere-year
Aluminum Anodes: 7.6 lbs/ampere-year

Regarding pricing, the prices I quoted that compared aluminum and zinc simply illustrated that they are not that different between them.
I'm sure one can shop around for better pricing though boatzincs.com seems quite competitive.
I'd really want to have a trustworthy manufacturer for either zinc or aluminum anodes. Small-time Chinese manufacturers are notorious for substituting in cheaper materials with a resulting performance loss.

I'll stick with name brand anodes advertised as mil-spec.

In a private communication, Nigel Calder gave the nod to my scheme of using an aluminum grouper at the dock, thus sparing the hull zinc and prop zinc. I am overprotecting a bit but no problem on a fiberglass boat. We'll see how it goes.