Maybe I am misreading here, but it seems to me the question was the other way round. Can you press the body in the shape of the (rigid) prop?
The cloth room assumes the flexibility to adapt is on the side of the cloth. The body vertices are not affected by the cloth.
If you want a tight perspex corset that squeezes the fleshy parts and makes them bellow at the perimeter of the corset you may have to go a long way. What you are looking for is internal pressure; something not provided by Poser (not by Poser 5 at least).
Proper modelling of that phenomenon requires re-meshing of the edge area (unless the form of your corset matches the meshing of the body, of course).
One free tool able to simulate surface tension, internal pressure and gravity at the same time is Surface Evolver:
http://www.susqu.edu/facstaff/b/brakke/evolver/

I have played around with an earlier version a long time ago, and I think it could be possible to use it for the intended purpose, provided you sit down and wite routines in C to establish collision of the body vertices with the corset, and utilities to convert the .obj files.....