Is this what you were trying to accomplish?

I applied a transmap to a simple square prop, and set the min/max transparency settings to 100%. Then I parented the square to the spotlight, so that it moves with the light.

By the way, that isn't the ground plane in my images. It's another square prop, with the ground-plane texture applied to it. For some strange reason, my ground plane won't properly display the shadows of transparent objects. As far as it's concerned, they're always opaque. So I have to create my own ground prop if I want the shadows (or lack thereof) to show properly.