
Currently I'm making such alterations by creating new furniture zip files and changing the mtl files (setting the d value to zero).
Additionally, if transparency was a slider it would be possible to have furniture items with panels that could be turned from solid, to glass, to invisible. It would would also help with creating door and window models inside SweetHome3D by combining boxes, cylinders, and triangles (which the new edit-within-group in beta-5 already makes much easier).