Any chance you could add the 'library search' option for Overdrive?
I'm envisioning a search using this form:
http://search.overdrive.com/
Then the results page you'd just take any results that are epub, mobipocket, or pdf and also have a 'Find at a library' button.
THEN once that library button is clicked, Get Books would automatically enter the user's zip code (you'd need to have a user preference setting created for this). And the output of THAT action is what would appear in Calibre.
Is this too far out of the 'get books' scope?