My original question was to determine if I could use the aluminium frames, steel axle and steel bearing (all conductive material) as "common" (GND), thereby freeing up channels on the slip rings.
You can't have a path to ground through the bearings. Rest assured as pointed out, really bad things will happen.
The problem with connecting through the bearings is that they have only 2 small points of rolling contact. When(not if) it rolls over a tiny piece of grit contact is lost and then re-established. this causes arcing further degrading contact. If you looked at the signal with an oscilloscope you would see constant interruptions in the signal that get worse over time.
I hope this helps you to understand what everyone is trying to tell you.