Daz - I did suggest a possible way to do it
Originally Posted by BetterRed
Maybe one way to do it would be to extract the opf from from the MOBI with the MobiUnpack plugin and then use the from-opf option of the ebook-meta command to update the Calibre metadata with what's in the opf file - see http://manual.calibre-ebook.com/cli/ebook-meta.html
. I think the MobiUnpack plug-in is a wrapper around a command line program, if you used the latter should make it simpler to write a script
Did you try it
If it works on a single book then it should be possible to write a script to execute the mobiunpack and ebook-meta commands on each book in your library to extract the metadata out of the mobi files and write it into the calibre database. As I read the from-opf option you should be able to select the metadata items you want - ie you wouldn't select the author and title.
Sorry, I can't help with the scripting - that bit of my brain died years ago.