"Defacing" bills is against US law but it hasn't really been enforced: http://en.wikipedia.org/wiki/Where%27s_George%3F#Controversy

I'm afraid blatantly encouraging individuals to do this would give the Secret Service a reason to prosecute bitcoiners, even if it's a flimsy one.

I still prefer keeping it very simple.

Person finds bill with URL, visits website, enters serial numbers, possibly wins BTC.
Person learns that even unmarked bills may unlock BTC.
Person visits site often to check bills, investigates the BTC market and becomes immersed in the bitcoin community.
Person becomes an advocate of bitcoin themselves and begins proselytizing and spreading the word with bills marked with the URL.

Contests over how many bills you've marked, how far they've traveled, etc should just be gravy.