Effects Process after Opaque queue before Transparency?

I want to apply an effect similar to fog but it also involves color shifting based on world position and other factors. I can do this as a post process effect,(using depth buffer to regenerate world space pos) but then all semi-transparent objects (particles and other alpha blended objects) have the wrong position. (and thus effect) (since they do not write to the depth buffer they are always positioned at the opaque object behind them)

So is there a way to have an effect applied similar to the post process ones but have it happen before the transparent objects are rendered? (I can deal with the particles and such not having the effect applied)

An alternate method I thought of would be to have an object in front of the camera that is rendered at Opaque+800 (after opaque and before transparent) with ZWrite off that could read/alter the pixel behind it. While i know how to alphablend something on top, I dont know how I could read the pixel behind to modify it. Is the current render buffer available in a shader like the depth buffer?