Is this the next logical step? I was surprised by the torque efficiency of the hypocycloidal gear I made in the Hesitation, Repetition and Deviation log, so perhaps it's worth pursuing for a bit. Until today, in conversation with @Dan Royer I hadn't realised that a possible advantage of these hypocycloidal and strain wave gears is that when you want to DIY one you can design it so the output and fixed part are transferring torque relatively far from the axis compared to  a gear with a conventional shaft output. 

The setup above is just a concept tester, made with a 210mm (70 tooth) HTD 3M continuous belt, constrained within a printed PLA ring that has a toothed recess in the base to hold the belt in place and prevent it from rotating, relative to the outer ring. The back face of the outer ring is bolted to the NEMA 17 stepper motor housing in the usual four holes.

The belt teeth take the place of the ring pins in a conventional hypocycloidal gear. The cycloidal disc has 69 teeth, hence this is a 69:1 gear ratio.

This could be scaled up to a 100:1 with a belt length of 303mm and a diameter around 100mm. 100:1 would, if the components handle the forces, get the baseline NEMA 17 with 59 Ncm holding torque to the order of 20 Nm being targeted by #5+ Axis Robot Arm Study No.5. If the parts can't take it (which I imagine they won't) we could step up to HTD 5M and the diameter of the belt would be around 160mm.

Promise I'll get round to showing the strain wave gears soon. Let me know what you think of this approach in the comments.