Okay, first data points on the Simpleton design:
200mm x 200mm x 200mm working volume, give or take.
400mm x 400mm desk footprint, but does want to drop over the edge a bit.
Tests with an electric drill, while frankly terrifying, confirmed that things will rotate in a stable manner at speed. I'm only expecting to see 20mm/s out of this, which is nigh on 1,000RPM for the M8 shaft, though only about 300RPM for the actual stepper motor. For comparison, a Mendel X axis is typically doing about 100RPM.
This all dictates that I'm going to need an Arduino Mega to flip the steps fast enough, and that I'll not be doing much in the way of micro-stepping. This is cool: It raises the possibility of cheap and simple stepper motor drivers for those building "in the wild."
No pictures today as the thing is in bits: I'm replacing the Z motor corner. The only screws involved are the ones that screw in to the Z motor itself. The rest is done by mallet. Hammerrrrrrr!