lack of occlusion: as well as a ground shadow plane, we need an invisible plane or surface, parented to object or character X, which would prevent Poser from rendering those parts of X which are on the wrong side of the plane.

I think you may be misinterpreting what hborre meant by occlusion. All the parts of X should still be visible (they would only be invisible if buried in the 'ground'), but I think hborre meant there there should be some extra depth of shadow where X's feet are close to or touching the ground, for added 'believability'. (I'm not sure AO would do that on a shadow-catch only surface - I'd have to try it).

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)