I'm trying to render out a scene where I have used an area light and attached a texture as a gobo. I have setup the scene with some geometry and a plane with a RS Object tag so I can render the image with a transparent background whilst still displaying the effects of the gobo. I'd like to composite this in After Effects, however the gobo doesn't appear in the alpha channel when I go to render and I can't work out why.

This shows what I'm trying to achieve in the alpha channel, however this is the RGB of the beauty pass.

This shows the current alpha channel with a shadow matte applied to the plane. As you can see the detail of the gobo isn't visible here.

Thanks in advance for any help.