You could always render as a series of stills (that's a good idea in any case because you can restart it from the last scene if it crashes or something) and save those as PSD, TIFF or PNG, all of which contains the alpha channel. Poser's usual grey background is an alpha channel by default. Whenever that's visible, it can be used as an alpha mask.

FREEBIES! | My Gallery | My Store | My FB | Tumblr |
You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.