I’ll have a video up soon showing off what I’m doing. 
Basically it is an alien growth that is dynamically generated throughout the level (kind of like what Natural Selection 2 does).
Right now it’s just in prototyping stages (2 days of work), and will more likely change how it is done later on for performance reasons, but the decals generate where the growth spawns (they are like green blobs with a decal underneath them).
The decal later on will eventually have gaps in the opacity mask to give a veiny like appearance so only bits of the light will show even when the decal is covering the surface of the emissive.
The good thing currently is that the blobs are set up to destroy the lights if they are nearby, and basically turn the emissive off (0), but if blood splatters on the light or some other decal appears on the light, I don’t want emissive to override the decal, like you see in that first image, and in your example.
I also tried placing a poly plane on the mesh just underneath the light with a transparent material, but this does nothing.
I also tried to set the plane to have an opaque material (no opacity) and make it so it was just hidden in game, but obviously that wasn’t going to work…and it didn’t 
I tried mucking around with the blend settings to no avail, tried a bunch of different things within the material itself to no avail.
I am wondering if maybe in the material editor, there is a node that gets a sample of the world around the material and applies that to the material or something. Even if there was a node like this, it would be pretty expensive in performance I would imagine.