I'm no expert but ....
- Back emf is tough to detect reliably for slow moving things like this.
- Current isn't hard to detect, but for steppers isn't a good indication of stall since they operate at low speed and continuous high current anyway.
Other ideas....
- Sacrificial mechanical link?
- Detect torque mechanically? (Easiest way to do this is to let the stepper flex and detect how far it moves)
- Position feedback? (optical encoder - compare where you think it is to where it really is - cut out if too far out of alignment).