I need to render diffuse, shadow, spec., and z-depth passes; at least, of some trees. For later compositing.
Where is the multi-pass controls?

 They are highly requested for the next version...

But you can accomplish some of what you need with changing shaders (changing everything to flat white for the shadows, making everything 50% grey for specularity), changing render settings (not render shadows is some kind of diffuse pass, is it?) and use the g-buffers (for the z-depth you could take the 'distance' buffer).

It is possible, but it takes a bit of work. A bit too much, that is why everybody is requesting this for Carrara 6.

Hope this helps!

Thanks.
I'll just export and render w/ mental ray.
I wanted to do it w/ C because the exported meshes are over 800,000 polygons...

 You can do Z-Depth with the G-Buffers in the Carrara Render room if you want.

Mark