Maybe you could post why you'd like to do something like this and we could come up with a different way to achieve a similar end result?
how about when someone posts pics from somewhere you don't want ? or spams porno pics ?? apart from initiating user ban or similar i would still like to be able to word censor any posted item, be it word or link.
say i have numerous users with signatures that all link from a particular site and that site asks me to not let the users hotlink those pics. now instead of asking each user, say several hundred to change their sigs, i just wordcensor part of the link and their signature pics no longer work.