Most people don't care about that, so you get a gold star.
I doubt anyone who uses a song for a skit or walk-on gets permission. Chances are that there isn't going to be a secret IP agent in the audience waiting to jump on the first person who uses a licensed song. It's not worth worrying about.
Besides, if there is someone trying to spring a lawsuit, it'll be against the con, not you. They'd probably be closed down for the anime viewings and pretty much anything fun you see at a convention. While they're at it... Cosplayers arrested left and right, artists in the alley having their wares taken into custody... Copyright's a strange thing.
facebook page // nh/mass cosplay events
Next event: ???
COSPLAYS IN PROGRESS
Actively conctructing : Zell, Soren, Zexion
Back burner : Absol gijinka, Lie Ren
Coming next : Cloud, Kafei