If he weren't such an important part of the offense, I would imagine they would even put the QB in on onside kick situations as well. All they want them to do is to catch the damn ball and not let go of it. Punters, kick holders, back up corner backs and safeties, backup wide receivers, RB's who catch well and tightends, pretty much anyone who is good at catching and hanging onto the ball would be good candidates for onside kick receiving duties if I was the coach.