Like all PostMagic effects, LUT is a post-effect. That means its applied after the image is already rendered. The reason its not applied to UI is that PostMagic does reconfigure VaM's render pipeline to render UI separately and merge it on top of the final image.
Technically you could assign your atom to another render layer and reconfigure VaM to render that separately, too. However, you would get trouble with depth-order of 3D objects. Like stuff in front your atom might not actually be rendered in front, but in the back instead.
That said, LUT stands for "look-up-table". It's a plain stupid texture saying "this color should actually be this color". So, make your own LUT. Just do a screenshot of your scene, go to...