By combining different passes with different ZBrush materials you can get any effect you like. Just remember that you should be a close friend of Photoshop also. To export the passes into Photoshop I use the ZApplink plugin. When using ZApplink you'll get some extra "bonuses" in Photoshop, like all the masks ZDepth channel and the rendering itself will be separated into three different layers (ZBrush shading layer, polypainting layer and Z fill shading layer).
If you need to apply different materials to one model, the best idea is probably just to polypaint the parts that you need to separate with different colors. I think this approach is much more flexible then painting the different material son the model. If you do color painting you'll get an easy way to select these parts in Photoshop and also get any kind of material you like using passes (Fig.12).
ZDepth is great for composing. You can use this channel to get "cheap" DOF or a kind of "Fog effect" in Photoshop. To get a DOF effect I used the standard Photoshop filter "lens blur" and just loaded my depth information in there and selected my focal point... it's a really fast and easy way.
I also like to overlay different textures over the surfaces to add some dirt or rust and paint different highlights to add some rim light effects. These effects can be seen in Fig.13 & Fig.14.
My final image looks like this (Fig.15).
If you have any questions feel free to contact me via email.