Suggestion: provide premade vertex shaders for PixelEffects
There are a few situations where PixelEffects would benefit from having a vertex shader which passed along the world-space position as a varying.
Perhaps if the PixelEffect pre-processor detects "varying worldpos;" or something similar then it could activate a pre-made vertex shader which would pass the world position along as a varying (varying = interpolated across the fragments).
There may be one or two other vertex shaders which might be useful but I don't know them yet. :P