Matt -
The person you are angry at and rightfully so - is your ex. WHOEVER he wound up with - might be an ass. Might be unscrupulous. But that person "owed" you nothing. Ultimately, it was your ex who screwed things up. This is/was between you and him and no one else...
That being said - I think your shirt idea is cute... but ultimately - I don't think you want to walk around advertising a battle wound - no matter how big it might be. Another cliche is that the real best revenge is to go on and live your life as if that person doesn't even matter. That's not easy to do now. I know. I understand that. But do you really want to give him the "power" over you and your emotions?
He already took his part of the relationship away from you. Let that be enough. Don't let him take ANYTHING else. He doesn't deserve it. And you deserve more.
"A little nonsense now and then is relished by the wisest men" - Willy Wonka