The Kindle creates mbp files, too. As far as anyone can tell, it really is a mobipocket reader, with Amazon mobipocket files having the funky AZW extension.
I didn't look in them for metadata, since I didn't have a handy mbp editor hanging out.
There is no exposed way to edit the metadata "cache" though. Restamping the books with mobi2mobi --author "last, first" did what I wanted.
I have some batch files quick and dirtied now that will go through a tree of book files and stamp all the authors based on the directory names they're in. Once/if I get those cleaned up a little, I'll upload.