Coming back to this, I noticed something interesting today: Lumen appears to ignore emissive translucency for screen traces, except when TSR is set as the antialiasing method.
I’m not sure if this is a bug (and if so… which is bugged? Lumen? TSR? TAA/FXAA? No clue what the intended behavior is), but might be helpful in your case. Recorded here in 5.2 preview, but I also tested in 5.1.1 and confirmed the same behavior.