I don't know if this works in C-Script, but with Lite-C you don't need screen-aligned quads. You can actually post-process a screen very easily (look up PROCESS_TARGET), and the shader is given useful parameters such as the size of a pixel.

You can also very easily link stages together this way. Look up "stage" in the manual. You'll like what you see. The shader workshop also has a couple of really good examples of its usefulness.

BTW, in Aaaaaetc (I was playing it yesterday, and thoroughly enjoying it!), I still get a small floating quad during some of the videos that floats a distance in front of the screen with black edges -- it looks very much like it is meant to be a screen-aligned quad, but is too far forward.

Jibb

PS: If you really can't move on from C-Script, and if you haven't done so already you could set up your pp-shader to texture the screen-aligned quad in view-space, instead of using its texture co-ordinates. That way, the quad doesn't have to be perfectly aligned with the screen, as long as it covers it completely.