It has been shown a few times he can't really ride with anyone else. Furthermore, the adult staff knows more or less what's going on with Hiro, so logically they shouldn't allow Hiro to ride with Ichigo, at least not as long as Ichigo can still fight. If Ichigo became useless, then they might allow it, just to see what could happen. However, I do hope Hiro would simply refuse. He's supposed to fly only with Zero Two.