Unless your stepper motor deliberately provides a torque output, I doubt that it will be possible to determine torque just from the stepper drive side. In any case, I would have thought that inertial loads would be high relative to the loads you'd need to detect to stop crushing somebody, and measuring motor torque is not the ideal approach for that. If you really need to go with this then I would think that a sprung switch mechanically connected to the motor so that the torque of the motor operated the switch would be the best way to detect excessive torque.
A light curtain or contact sensor along the leading edge seems like a much better approach, if you can do that.