I have an RBD sim of an object that blasts apart in to pieces with a pyro fireball spreading from its centre.

I would like to export this as exr passes and be able to composite the pyro fireball and the exploding pieces as separate passes for more control. I believe Houdini has a method that allows "deep compositing" in Nuke using deep camera maps, which seem to be depth passes that allow you to make the pyro exhibit greater density the further back the object/pieces are within it for more realism.

If this is correct, how do you apply the DCM in Nuke, which in this case would be a depth map of the fractured pieces? Do you apply it to the pyro, and if so do you use it to adjust the opacity so that pieces further back are covered by "denser" pyro?

I can't find anything on this. Also, I believe there is a simpler method of using Forced Matte/Objects (and Phantom?) which does not give the depth of field accuracy, but will matte out the pieces to allow for compositing.