Shadow pass using Mental Ray
 

I've been experimenting with both compositing and global illumination seperatly and decided to try combining the two to do a realistic 3d composite onto a photograph. The problem is, Mental Ray doesn't appear to be paying attention to my render pass layers. I've checked and rechecked my settings making sure my "shadow catcher" objects have their beauty pass turned off and their shadow pass turned on, but when I do a batch render all I get is a single beauty pass from what appears to be the DefaultRenderer layer pass.

What do I need to do to get Mental Ray rendering seperate passes? Manually rendering out layers using the rather "hack-ish" techniques I've read about would be incredibly time consuming.