You are allowed to jump out if the hog is stuck with a nade, and I would say this falls into the same category. It's rare you see a rocket coming from a long way off AND are stuck on ice and KNOW you won't be able to get moving and the rocket is 100% going to hit you. But I don't see anything wrong with hopping out, and getting right back in after the rocket does its thing.
If your driver is killed, it might be better to die with him and hopefully spawn together and pair up again. As opposed to jumping out, watching your hog rocketed across the map and having to walk after it, wondering if you'll get there before it respawns.
