That's not really the point though, is it. Ok, there's 3 scenarios that could happen.
A) He's unbelievably good.
B) He's ok but forgettable.
C) He's awful.
A is unlikely, but even if it happens, it'll still be about the fact that he's "justin bieber", not about the show. B is probable, but it would beg the question, what was the point? It's still stunt casting! The show gets nothing from it, except the erosion of its integrity. Even if he's amazing, people will still only be watching because of a publicity stunt that'll wear off, and the regular fans will respect the show less. Lose lose. Would the storyline really be undermined if Justin Bieber is not in the role? Would it be any worse if some unknown young actor played it?