This script is based off the Depth of Field as a Post Process tutorial by Kris Kapp, which in turn was based of Tom Kluyskens' work.
It creates a lighting rig for Z depth using the near and far clipping planes of a given cam, and allows different color values to be input (other than the default black and white values maya uses). It works for Geometry as well as Paint Effects, and maintains any existing transparency in the scene objects.
The interface also allows the values of the clipping planes and the color inputs of the Zramp to be keyframed. To use, save the script as a shelf item, select the render cam and click. Adjust the values of the camera in another perspective camera (It's pretty hard to do it in the render cam itself). It is recommended that you save the scene as an alternate file as the script will alter all existing shader and PFX attributes for its purposes.
For further information, the documentation is included within the mel script header.
For comments and suggestions, email me at email@example.com
Minor update. Fixed a bug which gave the wrong transforms on the controllers when the render cam was scaled. Sorry about that.