If you look at the Code for what it is, it is really a safety device. However, much of any such wording is impractical or irrelevant in certain situations.
In this case, you could figure any modification, at all, to a mechanically produced part violates the listing and is therefore taboo. However, we routinely punch our own holes in panels and boxes.
So the thing to do is ask yourself: "Is this compromising safety?" The hole you punch is the right size and in the appropriate location. Better, in fact, than what the factory can do.
Now you mention that this is for a recptacle. The Code is mainly concerned with making sure that there are no un-occupied holes and that the panel is not overloaded. Are you going to punch those two, semi-circular holes in the panel cover? I would suggest punching a big hole for the whole shibob and using a receptacle plate.
Or you could chicken out and just add an external handy box for the recept.
~Peter
