Most of the suggestions here make sense. Again it points to the basic limitations of NWN. If only, if only, Aurora allowed multiple textures to be applied such as those used in Skyrim and/or the way they were handled in Witcher etc...
The way we accomplished the look in CTP_Babylon was to add that extra layer to the object via 3dsmax/Gmax by adding an additional face whereby a transparent texture could be applied.... This was done on the building sections that were created by OMB and really looked amazing... the issue though was the immense amount of labor to duplicate so many faces and move them .1cm (point 1), out from the actual building walls etc... it works, but it does take a good hand at mapping textures properly and the texture itself must be large enough (1024x1024 or higher) AND be created in such a way as to hide or eliminate the tiling aspect, which is why you want to start with a 1024x1024 sized texture.
Of course, the issue arises that the total amount of vid ram used becomes soooooooo slow in NWN that the look becomes worthless when it takes too long to load an area that folks just want to get in and out of a given area in a mod so that they don't have to deal with the delay. That delay still exists even on quad core computers with dual sli vid cards and lots of ram since the Aurora engine just was not designed to handle such a load.
It works, but you pay for it with load times, and delays while running/walking through areas etc... however, most times, if you use such layering gently enough, you can fool the eye without causing pain to the inborn time sense we all deal with.