Well perhaps a silly question, but if you bought the Bader arm for the small wheel attachment, would it fit into the receiver of the NWG?
I'm not sure how hard the big peg on the Bader attachment is, it might be too hard to drill and tap. But you still have rotational forces to deal with, the pegs will be flopping around inside the thin walled tube. You can solve this by filling the tube with a tightly fitting block of hardwood...then drilling your holes....or welding bushings into the tube, which might be a bit more trouble. Also, instead of going to the trouble of dealing with the big peg, why not replace the small peg with a bolt going all the way through the tube and mount? The small peg on mine is a 1/4" roll pin so it seems this would be an easy swap. The only issue I can see would be tracking, if having the thickness of the mount on the side of the tube (rather than in a notch like on the Bader arm) takes the small wheel out-of-plane from the drive and idler wheels