Capturing emissive mask/setting up second temporary scene

I’m trying to capture an emissive mask of a texture via a post-process material and a render target. I have a setup that works, but it’s highly dependent on the amount of lighting in the scene.

I also set the material to replace the tonemapper.

So this works but it’s affected by any directional lights in the scene, requiring me to raise the threshold. With a directional light of power 10 I have to set the threshold to 5.0 to get the best result, if any more lighting comes in then non-emissive parts of the material start appearing in the mask.

So I guess my question is - is there a way to construct a temporary scene OUTSIDE of the normal scene? One where I can completely control the lighting and actors present.

If I can’t do that I guess I have to poll the scene and adjust the threshold to reflect the directional lighting preset.