So I had a long talk with my dad today about grounding. He seems to think that grounding anything post reg/rec is a bad idea. His logic is that after the 12v light coil and reg/rec any juice flowing through those wires will be 12 volt direct current. Since the 6 volt alternating current ignition is grounded to the motor/frame, it's a bad idea to also ground the 12v d/c lights to the motor, stator, or frame. So he suggests an independent, non-grounded circuit for the 12v d/c lights. I.e. send the black wires back from the lights to the black wire on the reg/rec. The lights will not be grounded, but the circuit will be complete.
I'm inclined to trust his opinion because he has worked as an electrician professionally and has built a few old cars. But I wanted to run it past the moped experts. Let me know what you think.