Besides the fact that the "Auxiliary Render Data" control is gone from my Render Settings panel, now that I've found Dimension3D's script to get it back, I see that the resulting z-depth map is badly aliased, making it almost unusable for masking. Is there any way to get an anti-aliased map, or do I just have to make a very large render in order to get smooth edges when I scale it back down to the size I wanted?

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If the link above worked for you, great. And in any case, feel free to ignore my late reply: Since every grade between white and black is a designation of depth anti-aliased z-maps aren't necessarily functional. Your foreground figure's shoulders for example, if antialiased each grey to dark grey pixel would connote a rapidly retreating value into Z beyond the actual position of the character. This is probably made worse including the floor plane as part of the depth solution. But you want all that if you want the z-map for a faster DOF render rather than doing it in Poser. My recommendation would be just do a separate render without background in order to get just the alphas of the characters. If its a long or complex render, copy your scene, delete all lights, turn off raytracing and render. This way you get the alpha for the cheapest render time. Sorry for the wordiness.