Can the areas of an object in shadow be accessed in Cycles In Cycles can I use the amount of shadow cast onto an object as a Fac. input of a mix shader?

This would be useful for setting up a Frost / snow material that is visible in shadowed areas but has melted in lit areas.

I know I could do it by baking shadow maps for the various objects, but I wondered if there was a procedural way to do it.

Maybe if I could create a vertex paint layer using the shadow info this could be used and would save the texturing (my scene is already quite complex - several hours per frame)

Any thoughts?
No, this is not possible in cycles.

As a workaound, you could render a diffuse pass in a different renderlayer and then use that as the mix factor, see Is it possible to use the output of a Renderlayer in the material nodes of another RenderLayer?