All you guys saying this lacks class has no idea what a rivalry is. The best part about rivalries is the bashing and games and pranks against each other.
Did no one go to High School? Did you not deface the opposing High Schools mascot or Rock or whatever? Its all fun and games.
Right now, SF is our Rival, and if everything works the way it should, for a long time to come. Buying a brick and having it say #12, or Go Hawks, or whatever, is nothing more then rival fans having fun. And it doesnt even deface your brand new stadium as its a brick, and its in place like all the other bricks and would take forever for someone to even see that brick unless they were looking for it.