Just go up to WD before the show and tell him that you can sing (make sure that you actually CAN), and ask him if they're playing DNB that night and tell him that you'd like to do the female vocal parts. Jodi did it in Cleveland one night and Warrel said it was nice to not have to do the female parts for the song. The girls aren't planted there, they're just fans who got a great opportunity. Damn, I wish I could sing.