Boy, if we could only get discreet and A|W to help us with this one.
Yes, combustion only supports z-depth compositing using RPF files. As far as I know, the only app that renders RPF files is 3ds max. For a while discreet reps were telling everyone that you can do z-depth compositing in combustion using RPF's predecessor, RLA, generated from your favorite 3D app. So just render to RLA out of Maya, right? Wrong. After each companies' techs pointed their finger at the other, I concluded, (with some acknowledgement from A|W) that the RLA files written out of Maya do not contain a z-depth channel that is compatible with combustion. Too bad for us.
What would really be better is if the combustion user could define an image to be used as the z-depth channel of another image. That would allow us to render a pass of high-quality grayscale images to be used as a z-depth channel on an RGBA image.
Hopefully they are listening.