Strange... using the simple scene of boxes, I got this to work using P4-renderer as you describe.  However, when I try a complex scene with multiple figures & props, it returns to being just a b/w mask with no grey scales.  All the guides are off, and the depth cue is on.  

I should say, however, that I did get one patch of grey at first, and that was in the figures eyes, but when I put on a different eye texture (a simpler one w/o shaders), that turned to white in the mask.

Any ideas?