Ways to render in Zbrush
If you do your render in zbrush (BPR) you can export multiple images and comp them in photoshop.
Just an addition...(Awesome Tutorial)
Tutorial by Vitaly Bulgarov
And understanding of BPR
In mental ray you ofcourse have more flexibility but results in Zbrush are also outstanding!
And you must look at some stuff from pstchoart
All the skin test renders are done in Zbrush
And Clothing,Hands and proportions need some fix!
Work on the overall silhouette and then proceed!