AFAIK, fog doesn't and can't work like that in NWN. If I understand what you're requesting, what you want would be "volumetric fog" and something like that would pretty much have to be done with a special shader. Otherwise, that's more the realm of rendered CGI rather than game art. However, you could certainly fake it with light halos, but those are going to have to be things you add into a scene by hand or to models. I think fxpa_dot1.tga or fxpa_flarel.tga attached to a light with the right settings (or possibly emitter settings) would allow it to be "colored" by the light it's attached to and you shouldn't have to deal with self-illumination issues. I mean, you can probably edit the settings of the existing models and/or TXI's you're already working with to at least get rid of the self-illuminated nature.
Moreso, you could probably mix dynamic (light) placeables and scripting to create some pretty nifty-looking dynamic scenes if you wanted.
Edit: FWIW, here's Sunjammer's package from the new Vault.