Forget for a moment that we're talking about people whose beliefs we (presumably) strongly disagree with and imagine another scenario - say youth at a Democrats rally are given T-Shirts with Obama on them, take them home and find that after a wash they have a pro-T-Party slogan. I would think that pretty unlikely to change anyone's mind. They're more likely to be pissed off.
I can imagine the Exit people watching the concert and giggling at their clever prank. Cute prank, amusing, but as a technique for persuasion?