Wouldn’t the render target just clamp the color values between 0 and 1, though? There wouldn’t be any way to store the emissive color information, so it wouldn’t be possible to adjust the emissive levels of different images in the UI separately.
I think the only solution right now is just to not use widgets to render anything that requires post processing. Instead, it’s better just to spawn 2D sprites into the game world in front of the camera. This is quite a shame, since UMG is quite nice for editing a HUD or touch controls. Maybe if enough of us request for this feature to be added, Epic Games would get around to improving UMG and widgets. I know there’s another post (3D widgets affected by lighting - Rendering - Unreal Engine Forums) requesting for more control over post processing on 3D widgets, but it’s been years and they still haven’t added the feature.