I have a forum where that would actually be the case. How about if you could link the uploads of a forum to a particular category or album in the gallery (as a forum setting), so that you can set the appropriate permissions there?
I would like to force the images into the gallery, because I feel that is the appropriate area to have them, so I would disable or hide option to not link the image to the gallery.
By the way, can't a user set the permissions in its gallery itself?