As it so happens, I also have the task of learning about shaders hanging over my head like a dark cloud, so my deepest of sympathies.
I've never worked in Unity, but in my native homeland of Game Maker: Studio i'd probably approach your particular visualization of the echos with an application surface, where we essentially draw a big black shape over the room on its own special layer, and then poke holes in it as necessary. I know it's how a lot of GM:S games do things like fog of war and light halos, and it seems like they would suit this nicely.