Shader: Adding alpha

I have a shader material which I pulled from here: http://forum.unity3d.com/threads/31163-Atmospheric-Scattering-In-Unity-from-GPU-Gems-2, for atmospheric scattering. I am fairly new to shaders, and this is way over my head. I'm looking for a way to add an alpha layer/filter to this shader so that anything that is black, > an rgb of (20, 20, 20) is not rendered, transparent. Any shader guru's out there with some advice?

P.S. - I'm looking to do this with the SkyFromAtmosphere shader.