When I instructed in 172's, I supervised power-on stalls with a student in the left seat and the back seat. As long as the CG is within limits, you can perform power-on stalls. This was done both VFR and under the hood. Obviously, you need to keep the coordination/yaw in check though.
Oh, Lordy, I know you mean well but most 4 seat airplane hand books prohibit any stalls with the rear seat(s) occupied. It is rediculous I know but that is the way it is now.