The DS Shader Mixer has a Ray Is brick which can be used to control how a surface behaves according to the type of ray hitting it - I've been experimenting with that for things like lampshades, where I want some light to pass through without making it transparent and putting a camera behind a wall while still having the wall show up in reflections (I stilll have a puzzle to solve there). I don't see an equivalent in the docs for the Poser 6 Material Room, has something similar been added in 7 or 8?