Sometimes pure theory and soapbox legalism need to take a back seat. Our inspectors look at it this way: 1) have we made anything (even the slightest bit) less safe? 2) have we made anything (even the slightest bit) more safe? The answers are 1) no. we have not put any more load on the exsisting circuit nor have we placed any part of the circuit in a position to become less safe. 2) yes, tremendously. We have not made the system any safer for the building or eqpt but we have put the person in a much safer position. Therefore, assuming (yeah we know ass out of u and me)the new recep is not in a location that can be reached somewhat easily (like from an unfinished basement below) they look at this the same as a replacement, not as a new installation.
Let's look at this another way. If that customer had called you to repair/replace the light socket in the fixture would you do it? If you do, you have knowingly reinstalled a non GFI protected outlet in their bathroom. So now the guy has the option of being in the dark, or having no recep (because you can't reinstall the fixture) or spending lots of money for us to chop and patch his house up to get a circuit there.
Just my 2 cents. That's how we look at it around here.
