Get access to previous pass from CG

I wanted to use the built-in shader passes, (SPECULAR/BASE, custom stuff, etc.) and then continue to process the pixels in another pass, say blurring or some other effect. I can do something sort of like this in shaderlab with SetTexture[ combine previous texture ] which clearly accesses the result of the previous SetTexture operation. Is there a way to do that for passes? I would imagine there's some _Previous sampler2D or such that gets written to but I couldn't find anything like that in the docs. I'd like to do:

SubShader {

    UsePass "SPECULAR/BASE"
    Pass {
CGINCLUDE
        // do something with result of previous pass
ENDCG
    }
}

Check out the Blend command:

http://unity3d.com/support/documentation/Components/SL-Blend.html

did you try this?

remember that it’s much more expensive then make all calculations in a single pass