I have no explanation, but it looks like only those two sheets were at issue. After the one test sheet already mentioned, I did two more---one from each end of the stack---and they developed clear, so I took two more out and actually shot them, and the resulting negatives look perfectly normal.

Try as I might, I can't think of any way that I could have gotten two entire sheets uniformly fogged in a known-good holder. (Even if I'd pulled the wrong darkslide twice in a row, and managed to put them both back with the correct orientation but without noticing anything was wrong, the holder edges would still be there.) I guess it's just possible that something mysterious happened in my darkroom, but I really do think the simplest explanation is that my box had two fogged sheets in it!

Science is baffled, but I guess this is the end of the story. The best explanations I can think of are that it was X-rayed in the post and the film is radiopaque enough that it only affected two sheets, or that I happened to get the very tail (or head) of a bad production batch. But neither of those is all that plausible, and I don't really see any way to investigate further, so...*shrug*.