This is actually pretty simple. The unwrap is just a vertex shader using the lightmap UVs. You can visualize it by animating a parameter to disable the unwrap:
Then for the emissive color you can either use worldposition or local position. Worldposition is easier since you won’t have to transform your brush location, but it will require an HDR format and take more memory. In this example I used local 0-1 UVW which can be stored in regular texture formats and you should be able to transform into the local space by doing an “inverse transform location” in blueprint. Make a simple test case first to see if they match up. Maybe just start with worldposition instead of local since its easier:
To save the unwrap you can just place a scene capture 2d, assign a render target and tell it to capture emissive in RGB. Then you just goto the render target in the content browser and right click and select “Create Static Texture”. Done.