I haven't seen this done. It's just a theory of mine. Considering my experience with shaders, I wouldn't be surprised if it doesn't work, or even if it is utterly rediculous, or if it has already been done.

This would give alot of control over how the lighting looks (faded, crisp line between light and dark, effects like bump mapping (though with less freedom) can be added right in at no extra cost, etc). Here is what I think.

Could shading be done with Cubic Environment Maps? You could make a cubemap containing the lighting detail you want. Couldn't this cubemap be used as a shadowmap for the entities using this shader? The artist could make a cubemap using gradients, maybe a bit of colour, apply generalised bumpmapping, make crisp lighting or fine shading, and have really fancy looking lighting for very low CPU cost.
I'm quite prepared for this to be shot right down, or someone say, "it's all been done before," but I've only ever seen Environment Maps used for chrome effects, specular effects and reflections.

I only have Extra edition, so I won't be able to try this out myself.

Julian