Is this something that default Unity shaders do? What do you mean by grime exactly, blending in a diffuse map, multiplying on a dirt map, AO in general? Marmoset Skyshop comes with full shader source, so you can add in your own features if you like(or you tech team can).
It is possible to have multiple skies in the same scene. It is also possible to assign custom sky lighting to each object individually (actually each material for now :-/), so one could write a script that assigns a sky to every object within a given area. If the object moves to the next area, a different sky is used.…