I can't help much, but since you don't seem to get an answer, I'll give that little bit of information I have.
I'm not working with C2, and my experiences are from HLSL and Ashley's implementation of pixelWidth/Height there, but since GLSL is very close to HLSL and there's probably no reason for Ashley to change the behavior of pixelWidth/Height (I'll omit 'pixelHeight' now), here's my two cents.
pixelWidth is nothing more than the width relative to the normalized display size. Or, in other words, pixelWidth = 1/display size and unnormalized display size = 1/pixelWidth.
You should be able to use smoothstep, since it is available from v1.3 up. The example, you linked to, contains an alternate method to aa when dFdx/dFdy are not available. It explains that you need to know the "window size, the view transformation, the gradient in texture space and the relation between texture coordinates and world coordinates".
Well, the texture coords are absolute values of the normalized world (display) coordinates, like 0.25, 0.25, 0.5, 0.5 (a texture half of the display size, centered on screen). You get the window (display) size from 1/pixelWidth, you should already know, how you transform the view (or expect it to be untransformed), but I don't exactly understand "gradient in texture space". Maybe that's the range of grey shades from the texture's colors? That's the last point you have to find out by yourself.
Maybe this doesn't help you, or you already knew. But better told twice than never ;)