3d render to alpha channel

Hi,

The idea is to make a shader that does this : http://www.youtube.com/watch?v=6r7hVWmNEcw&feature=g-upl

A friend of mine did this shader with HLSL ---------

 uniform extern texture ScreenTexture;    
     
     sampler ScreenS = sampler_state
     {
         Texture = <ScreenTexture>;    
     };
     
     float4 PixelShaderFunction(float2 texCoord: TEXCOORD0) : COLOR
     {
         float4 color = tex2D(ScreenS, texCoord);
     
         if(color.r < 1.0f)
         {
             return float4(0.0f, 0.0f, 0.0f, 0.0f);
         }
         else
         {
             return color;
         }
     }
     
     technique GreenToAlpha
     {
         pass Pass1
         {
             PixelShader = compile ps_2_0 PixelShaderFunction();
         }
     }

--------

Do you know the process that I can use with the free version of unity3D ?

Or maybe this shader have been already done in unity, if you have a link to it feel free to post it.

Sorry for my poor english.