Okay, I found how to see UUID (right click book title in calibre, view book details), but now I'm confused that MobiMetaEditor claims several books have no UUIDs, when I can see that they do in Calibre.
Just creating a cover file with the UUID isn't working - the kindle doesn't seem to register it as a cover. Think I need to follow the instructions
here and see if they work to get the covers on these last few books.
(Documenting what I'm doing for anyone who might find this thread in the future.)