poser ground plane has shadowcatch-only setting, but it's not very good IMVHO, and may not be useful with IDL-only renders.  bill's free shadowcatcher shader is much better for poser ground plane, but it will be difficult to learn in most cases, and it gives unexpected results when using hi-res jpg background and HDRI lite source on the sphere (e.g. sIBL method), due to the refractive nature of bill's shadowcatcher shader.