Lumen still picks up pixels from the screen, so really you can’t (this is a problem they’ve mentioned). Though, looking at the video in this post, it looks like it can be excluded from the lumen scene, but it still gets picked up by the screen.
However, you may able to “fake” emissive through a post process applied before tone mapping; just mask the emissive parts and set their color value to something bright. I don’t have UE5 installed right now, so I can’t test it.
Also, I thought emissive materials had no additional cost?