Z-Depth Compositing for Maya users?

[zqw]

First of all, have any Maya users been able to do any z-depth compositing using RLA, IFF, or any other method in Combustion?

In Combustion, is it possible to use the luminance of an RGB image as the depth channel for another image? If so, you could render out a black and white ramp that is projected onto your scene, making objects close to the camera white, and objects far away, black. Unlike RLA's or IFF's the result would be a fully anti-aliased image. But then can Combustion use this RGB image as a depth channel for another image???

Thanks for any info,

-Greg