Shader for object-specific ambient light?

I'm trying to juggle lighting for various scene elements in my space shooter. The problem is the background nebulas, which are layered quads. They use the Mobile/Particles/Additive shader and are rendered to their own orthographic camera. I'd rather control their brightness using ambient lighting, but increasing the global value throws everything else off. Instead, I tried a directional light pointed straight down, but that creates noticeable rings on some backgrounds.



Per-object ambient lighting would be my ideal solution - I don't suppose I can get close with a built-in shader? I tried a handful I thought might work, such as Mobile/Particles/Vertex Lit, but that one just killed transparency.