How do you convert Mesh/Material/Texture Nodes to Compositor Nodes?

Question

This is in regards to my question: "How to extend an Image from side to side softening the hard edge of image" How to extend an Image from side to side softening the hard edge of image

The file link on that page is https://blend-exchange.giantcowfilms.com/b/3584/

The Effect works good, but needs some tweaking that does not work in the compositor using this effect as an input, but using Mesh/Material/Texture Nodes, aka, selecting the Mesh and using a node editor, is that you are sending this data to the Compositor, and yet you have no access to nodes its using, its prepossessing the data, in this case I do not want to use any Material, I want to use the Compositor so I can use Difference Key and remove the black background, so the Stars show through around the Sun.

I could not get that to work in the above blend file, I know I can still do that, but it does not work the same, if I use an Image Sequencer and transform its location by scale of .5, and mix in the Environment Image map of Stars, then add this: lets call it a node group called Corona, from above file into a compositor, it does not work, see link below for this file.

My guess is that I am not using a Mapping Node and object input, and I do not know how to fix that.

I do not appear to have a upload option so I will post a link below to the file I am working on.

http://lightwizzard.com/resources/home/lightwizzard.com/Light-Wizzard-in-the-Flesh/Sun-Corona.blend.zip

What I want is the effect of making the Corona that moves outside of the image to have a tapered look as the first thread in link above tries to explain, note that if the Corona does not make a hard edge, like the second image in this zip file, then I do not want any effect added, so I only want to expand the pixels out that are actually going past the image bounds, maybe using the lighter red as a threshold, and only effecting the outer 1 px, if needed I can alpha over the original image such that its image has no effects added to it, I do not really want to change the original, just fake the Corona but make it look real, so it would still need to taper to the darker red color to look natural, hope this was enough information.

Thanks.