Had a go at this. My goal was to have a light cone that did not require any keyframing and worked perfectly with the iClone light tool. Here it is. All you need to do is set it directly below your light. It is transparent with high specular and a little gloss. The idea is that is totally passive, and just reflects light like dust in the air.
This is just a quickie built from an iClone cone prop, so it has a bottom that you really don't want. In this case, put the base of the cone just underground a touch. It could also use a bit of rotation animation on the Z axis if you wish.