If presented with an opportunity to believe that other people are like you, with no penalty for being wrong, one could expect people will err on the side of predicting behavior consistent with one's own behavior.

I obviously haven't done this experiment, but I suspect that if the subjects asked to wear the sign were offered a cash incentive based on their accuracy of prediction for others, both groups would make a more accurate prediction.

Possibly. But if you're prepared to bet that the bias would vanish in that context, that's a bet I'd take.