I am pretty sure he means a keyboard short cut for Paste cover art from clipboard (now hidden under the right click Cover art sub menu). I support that request.
The ability to just drag an image from Album Art Downloader and drop it on the cover art action window would also be nice.
At the OP: It is possible to have Album Art Downloader configured so one mouse click downloads the desired image to a Folder.jpg to the correct album folder, which can then be picked up automatically by MC. If you want the cover art embedded in the music file, this however currently involves making a menu selection in MC.