Just make mask renders for each before compositing. Render each, and then go render as mask for each. That'll give a nice clean alpha channel right there.  ...Oh yeah, almost forgot - make sure that whatever you're rendering a mask of is selected.

But you can do it that way if compositing is the only good option. :)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
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There might be something worth downloading.