@theducks
Does save to disk also work to insert a book jacket? I don't know, since I don't use that feature, but it doesn't sound likely, and that is what the OP
mainly wants...
On the other hand, Polish EPUB by default saves the original copy as ORIGINAL_EPUB, so you keep the original version without the metadata book jacket.
You can do both steps in one by using the command-line version, here:
http://manual.calibre-ebook.com/cli/ebook-polish.html
Use the format
Code:
ebook-polish [options] input_file [output_file]
and pass as an option, "--jacket" or the short form "-j"
It accepts /path/to/book and /output/path, so you will have to script something with the "calibredb list" command to pass the locations to your script and then to save each book to some meaningful location. Good luck with that!