It wasn't NWN; that's why I only had the question, as opposed to examples. I rather figured that if it were as easy to jack the code in NWN as it was with the various FPSs I've played, (Unreal, Quake, Doom) it would already have been done by our Architect long since, as has been done with the various scripting et al.
So... The C++(or whatever) that runs the core of NWN cannot be tampered with, written over, etc., but what allows so much customization in NWN is the way that most of the game mechanics (e.g. Precise Strike and most of the other things that would go on a character sheet with notes beside them) are programmed in a different way. Right?
Well, there's that. I wonder if NWN2 will be any different...