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 } }