Gah! So simple, I am embarrassed that I didn't think of that. Way to go,Tony!

I didn't have to wait forever, either -- four minutes and five seconds for two gigabytes, 3,675 files. Looks like about 600 of the directories did not have a .mobi file in them, those would be directories whose only contents were subdirectories that did contain the .mobis. Like... directory "John D. Macdonald", containing 21 subdirectories, each subdirectory with a Travis McGee book in it.

The process went quickly because I didn't move the files (which does take forever) but copied them instead. After I run them through Calibre, they'll be copied once again into the Calibre library, after which I can delete the other two copies.

Thank you for the simple solution to something I wasn't smart enough to figure out for myself.

tanstaafl.