Paul,
I would make that connection by drilling and taping 1/2-20 providing a lug and 2- 1/4 20 hex nuts to actually lock the connection tight to the threads and enclosure.
So the only metal to metal connection is the threads? There is no way that 1/4-20 threads have enough metal to metal contact in a field tapped hole to provide an adequate electrical connection. What is the gauge of the metal? The commonly used 10-32 screw only has ~2 threads engaged in field tapped holes in many types of electrical enclosures. The 1/4-20 would have less. If you don't clean the paint off of the enclosure this is the only current path. I don't see it as suitable.
Don
