They absolutely, without a doubt, would definitely have slow-danced at their wonderful golden-gardenia-garnished wedding. It would have been beautiful and perfect and Deb would have cried because Brian was finally showing Justin (and everyone) how much he loved him. It would have been like the prom with everyone in the room stopping and staring, but everyone would be crying with happiness (Deb, Em, Ted (he cried at Mel and Lindsay's wedding!), Jennifer, Daphne) or just staring open-mouthed in awe (Mel). And there wouldn't be any baseball bats.
In private I can see Brian and Justin dancing spontaneously in the second half of S3. If they had music on in the loft and Justin happened to sidle over to Brian and pull him up off the couch and started to hug him and sway a little... and if would be perfect too. Man, the end of season three... teh love is overflowing.
I agree with NY. I can't imagine a situation in which Brian ends up at an event where slow-dancing would take place (except maybe Deb and Carl's wedding), but if he somehow believably ended up at an event like that and if he felt there was anyone there who wouldn't want two guys to dance together then he would definitely slow dance with Justin then. And I think he would dance with Justin at Deb's wedding. She would make EVERY couple get up and dance.