Apologies to Hofferic for jumping in here.
@hardcover: You can do this with
Calibre Companion in two different ways.
The first: transfer empty books to CC as
described in this thread. From time to time you would connect CC to calibre as a wireless device and transfer any new books. Connecting automatically updates the metadata stored by CC.
The second: put your calibre library on an SD card and use
CC's "local library" connection feature. You could even go so far as to copy only calibre's metadata.db file, but in that case you would not see any covers because they are not in the database. You don't care that the books themselves are not there. I can imagine writing a script that copies only the metadata.db file, the directory tree, and the cover.jpg files.
If you want to discuss this further please post
on the CC forum. I don't want to hijack this thread any more than I already have.