This isn't intended as a solution but it can add a degree of realism to shadowed areas. Use two or more sun lights in the same position with varying amounts of Soft shadow. The top image below (using a model from the site mentioned above) is lit with three lights, one set to Soft Shadow = 0.5, the next set to Soft Shadow = 15 and the third set to Soft Shadow = 45, the light intensity is 1.5 on each.
The second image has an additional bluish light at 90deg elevation.
Other more varied settings will give more effects.
Richard
ps: Oh and don't forget to turn OFF Specular Highlights and Glow in Atmosphere for all lights that are NOT the main light source (unless of course you want multiple glows and highlights in your scene)