Removing the backfacing polys would still show the shoulder strap etc that would normally be blocked by the body.
I think what flashmoyo wants is an image that can be composited onto an existing image.
Eg: Picture or live footage of say short walls and objects, with a poser character walking behind them. Now a material that could be applied to a 3d object (stand-in) that would when rendered make everything behind it disappear "an alpha mask material". As far as I know poser can't do this, however

You could do it the way arcebus suggests which is probably better for Photoshop. Or depending on the size of the image or video take it into a video editing program with greenscreen filter.

Just thought, don't know  different poser rendering scripts etc, but there was one called glow worm that could supposedly render out different layers etc for adjustment later. It may have the ability to render out alpha masks or just selected layers eg the cloth that is not blocked by the body. I think they sold t at Daz3D.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches