Paul,
Possibly you require the removal of the Neutral bus and the paint sanded off although the UL listing and the code approves bonding through a screw and a tapped hole as long as it is not a sheet metal screw.
The hole(s) provided by the panel manufacturer for the connection of the main bonding jumper or the connection of the equipment bonding bus are not the same as a field tapped hole. The hole is punched first, providing a greater thickness to be tapped and better contact. That being said, I don't see how that is not a corrosion problem if removing the paint under a lug is a corrosion problem. Also if the interior of the panel is subject to corrosion, I think that there are much bigger problems than the corrosion of the can.
I don't see this any different than field cut threads on conduit where the factory applied corrosion protection has been removed by the act of threading the conduit. The code provides a provision for this and the same method could be used in the panel if there is a potential corrosion problem.
One more point on this, the orginal question is talking about a "large" conductor that won't fit on the provided neutral bar. To me that would require a screw larger than 10-32 and if the screw is larger the screw pitch is larger and the tapped threads in the can will provide very little contact.
Don
