My scene contains both Dmap and raytrace shadows, depending on the light. I'm doing the "use background" shader approach for the shadow pass, rendering with MR. The problem is that the lights using RT shadows aren't casting. All the basic check marks are in place (or so I would think - Maya 8.5's mental ray tab has a whole new set of Shadow settings which I hesitate to toy with).
How do I get my raytrace shadows to appear on that alpha channel?
Sorry to sidestep the question, but why do you need to use MR if you're just getting an alpha pass for the shadows? The Maya software renderer should produce the results you're looking for without any headaches.