for the record, I think this idea is stupid and immoral. Stupid because all they have to stop you doing this is change their .htaccess file which will take, oh, at least five seconds, and immoral because hotlinking is hotlinking and theft is theft.

I expect to be flamed for saying this, but if you really want to go after these people, do it in a legal, defensible way, or you're no better than they are. Two wrongs don't make a right, and in this case, the second wrong doesn't even make sense.