The way Brian was when the show ended; yea I could see them slow dancing together. More so alone than in front of people. His emotions were bared so openly and he was unafraid to show it. You could see all he wanted was to be close to Justin and because of that, I could easily imagine it happening. Bottom line - post-513 Brian would definitely slow dance.
As for Justin, there was never a time that I thought he wouldn't want to do that, even when he was with Ethan.
I always wanted to read a believable fic in which this happens. It's been one of my guilty pleasures since watching the ep. where Michael and Ben dance to REO Speedwagon.