I called the ANC location and, to my surprise, got what seemed to be a very knowledgable agent familiar with such rentals. She said I would only actually have to pay a $6-and-change daily facility charge -- not the roughly 19% "tax" on the fictious (and very high) rental cost.

Obviously, a much better deal, and I'm going to try it. Fingers crossed. While this is a "peak" award, it's still a good deal given the high summer prices. My only non-crazy alternatives are to use a dubious code with another rental agency.