I like both ideas.
#2 gave me an idea though, instead of a trigger in a song where they strip, instead a trigger in the song toggles their hypnosis, so either stops their trance or puts them back into a trance.
This song could work for that with the clapping (there's some parts where it claps with music, I was thinking just the parts where the music leads into the claps like at 1:02), I think it's called Solar System Disco https://ecchi.iwara.tv/videos/kp29dt7arghqjwjeq
So the first time they hear the clap in the song they become unhypnotized, find themselves exposed on stage and try to cover themselves, and then a moment later they hear the trigger and become hypnotized again and continue dancing exposed.
Or sticking with your idea, you could use this song as the trigger to strip where she sings "so cool" (I think? or is it "so good"?), I counted her saying it 4 times (0:11, 0:17, 1:08, 1:15) which works well for the amount of clothing (top/bottom/pantsu/bra): https://ecchi.iwara.tv/videos/0oqyksjjeizk7g1m