Well i suppose the only way to use it as background is to move the pic to the album.. because:
I suppose the background image function of vb
also checks the albumid, if the album belongs to that user, so users cannot use other peoples backgrounds etc... and permissions, is that usergroup allowed to use images from their album for background.. etc..
So if it is not in album, you probably wont be able to use it..
IF you have album turned off, I do not think the background function will work.. (unless for images allready uploaded before you turned it off)
IMHO, i think the only way of achieving this is by moving or copying a picture from vbgallery to the album.
As in a couple of months vb4 is going to come out, i do not think anyone will write something like that because it is very time consuming and will probably be obsolete by the end of the year
The easieast way to achieve what you want, is to write a tutorial for your users to rightclick an image.. save as on their computer and upload it to album (that you can turn on with restrictions) so they can use it as background...