I do not totally comply with you, Celladur. As far as it goes, if a file exists, it is moddable, given the right tools (even if that tool has to be a Hex-Editor). I've been messing around primarily with the ERF-Files (see TexturePacks Directory) and their content.
ERFs seem to have been virutally unchanged since NWN, only in minor ways, so it was really easy to make a custom tool to extract the interior files and be able to compile them back to an ERF. Not really a challenge, though it took me a damn day.
Now I'm messing around with the texture files, mostly those insinde "swpc_tex_gui.erf". They puzzle me a little bit at the moment, especially given diffrent filesizes in the same video mode (namely 800x600), but I'm still making progress. Hopefully I can uncover the exact structure of those files soon, my tries so far uncovered some usable info, but not really enough to start messing around with textures. I'm working on it though. In for the 4th day or so...
Somebody wanna pool their efforts with me, so we can figure out this stuff faster?