Tried something:
If a part of the material is always going to be glowing and reflect no light, we can use it’s color value as an emissive ID mask. For simplicity, we can treat each channel as a bit and get 2^3=8 IDs (2^4=16 with alpha and more with other textures). The ID can then be used to get the emission value from a much smaller emission texture; it can also be used with parameters or procedural effects, too.
As seen in the first picture, the emission texture is only 8 pixels (8x1), which is super small; it’s basically just a palette texture. Plus, this works without any additional setup on both the texture side and the material side: you just set your diffuse and emission textures and you’re done.