Ah, I remember why I'd been doing it this way -- with my older cameras there's no CR2 -- they just write out JPEGs directly so I'd been trying to back up the exif data without backing up the whole JPEG image.
Is it possible to restore from an HTML dump?
exiftool -htmldump -w ./EXIFBackup/%f%e.htm .
works fine for backup too but it seems to stop on restore. I know this must be something I'm doing . . . or not doing . . . :-D Or is there a way to (would it be easier to) pipe the output of exiftool -s to a set of text files and then pipe the text files' contents back into the jpegs?