While I’m about 4 years late, throwing this here for anyone else who needs it.
You can use SceneTexture:PostProcessInput0 to get the current scene. Then if you consider that emissive is basically just light that is above 1, you can subtract 1 from that to get the scenes emissive. You may also get some areas that just have bright lights, so you can increase the subtraction to reduce any non-emissives.
In the above example, I’m also turning it white (desaturation) and clamping it to 0-1, so I get a nice black+white mask of where emissive colors are used.