In my opinion all of you are flirting with disaster.
Most panels are not made of cans with thick enough metal to take the required two full threads when tapped.
Removing the paint to attach a lug is dangerous for the stated reason, and, the unreliablity of field installed lugs. (What nationally recognized testing labroratory has tested these connections and has detailed the steps to follow?)
The only grounding connections I would approve are the neutral or ground bus, the supplemental grounding bus kit (installed in the pre-formed holes, using the included self-tapping screws) or a grounding bushing.
