I believe Wyll's romance trigger is asking any sort of question about his dancing days. Not during the party itself, but rather the very first time he talks about his life after the Duke has been abducted.
Because even though I never flirted with him nor asked him if he wanted to dance at the party during my last playthrough, his scene still triggered in ACT 2. So I believe his trigger is asking him about his dancing days, though will have to test some more when I start a new playthrough.
