That’s one of the solutions I meant by “if they aren’t specially set up for that.”
The problem with that is that, every material I might want to put a “glow” on top of, has to be edited to support being “glowed.”
In the best of worlds, there would be a kind of material input that is a “material sampler” that takes another material expression as parameter (like how a texture sampler works) and outputs material attributes from that material.
In such a world, I could make my “add glow” material take the original material as a parameter, and thus don’t need to change/edit the base material.