Yeah did a tonne of research and trial with that mip_matteshadow today, it actually does not support final gather light bouncing when it's set to not render so it's kind of useless. My whole problem was I couldn't get the light bounce I was after and the alpha channel I wanted since the ground plane bouncing the light had to be the use background layer to capture the alpha correctly. I solved it with 2 render layers that were automatically composited using that render all layers composite and keep layers option.
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Edit: on this subject cgtalk might be a better forum for help
http://www.cgchannel.com/2010/07/tutorial-multipass-rendering-in-maya-and-mental-ray/
Thanks in advance, oggo