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.
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COSPLAYS IN PROGRESS
Canterbury (Kuroshitsuji) - 30% - Kuro Gathering
Utatane Piko - default (Vocaloid) - 15% - Ohayocon
Boris (Alice in the Country of Hearts) - 3%
Armin/Jean/Connie (Shingeki no Kyojin) - 0%