If you're planning to render outside Poser, like in Bryce, you may try that: Scale up a copy of yor model to the XZ size you want the deformation area in matress. After that, scale it down in Y to shallow the depth deformation. Export it and use as a negative object in a boolean, were the matress is the positive one. The borders may show some problems. Perhaps if you flatten even more the double in Y, and apply some magnets to give it that S shape, you may get a more natural effect.