I don't think there's much point in getting stuck on exact definitions and semantic subtleties, but the point is that a mod CAN add something that doesn't already exist in the game, or can CREATE nudity (or whatever other objectionable things).
To be technical about it, it can't be considered an easter egg since it couldn't have been unlocked, just as Marshal Murat pointed out.
And there's been lots of nude/gross/explicit mods for plenty of other games out there. Nude quake characters would be one of the more popular ones, I suppose.


Either way, it's not very important. The point is that IF this request of ESRB is actually implemented by game makers, it will do nothing but hinder the game industry, because:
1. it will stop game creators from making easily moddable games, a perfect example of this being the text files in RTW. Most nude/gore/whatever unacceptable mods do not come from this kind of moddability.
2. it will not stop people who reverse-engineer the games from creating mod-engines, or implementing the very things that the ESRB is concerned about.

So all in all, it would do nothing but hurt. My opinion, of course.
But then again, why would y'all expect that the decision factors in ESRB are more than simple bureaucrats, for whom logic is not exactly their strong point ? ...