Yeah I've played around with alpha masks and zdepth since bryce.  I use PhotoImpact.  It is a good way to add a type of DOF without the long render times.  You can also control the depth by changing the brightness and contrast of the zdepth alpha.  Mario, an expert Brycer, over at 3DCommune, wrote a tutorial of how you can make neat light rays by custom making an alpha mask.  For example you could render a scene normal.  Then duplicate the scene and save it under a different name.  Then create some long, stretched cones, apply a transparent material to them that look a bit like beams of light, then change the mats of everything else in the scene to solid black, and render again.  In your post program, add the new mask scene to the original render as an "inverse of multiply" layer.  Only the light parts in the mask will show up (aka. the light cones).  You could even add color to the mask layer to make the light rays yellow or blue, whatever.