Subsurface scattering works by specifying the desired color and radius on the surface, and then we figure out the appropriate volume scattering and absorption parameters that will produce the color and radius. That way you can texture it just like any other diffuse surface. We currently only have a numerical fit for the 0…0.9 range.
Possibly it could be extended, but this covers the most common types of materials that subsurface scattering is meant for. With a material that is very highly forward scattering, the light will mostly scatter towards the other side of the object and not come back to the surface. To compensate you have to make the volume parameters more and more extreme and it just stops making sense to think of the material in terms of surface color and radius.
Real world measurements for skin have anisotropy around 0.8, but the difference with 0.0 is quite subtle.