I'd love to see the scene if they took her to the police station for this. "Uuum I hate to tell you this, but fancy dress isn't illegal guys"
That happened in the DCAU comic tie-in "Batgirl Adventures." Harley gets Babs!Batgirl to chase her through Gotham, then gets her to help rescue Ivy from some terrorist group who wanted Ivy's formulas. Harley let Batgirl bring her (Harley) to the police afterward, but since she had been released from Arkham, they had to let her go.