I'm trying to make a material node setup to set final render colors, regardless of lighting conditions. Is such a setup possible?
I assume you mean a "shadeless" shader:
To create this you can use the Light Path node to make an emission shader which is visible only to the camera, letting other rays straight through with a transparent shader:
With a diffuse shader (or a holdout shader, black transparent shader, etc. anything that will cast a shadow and render fast) used instead of the white transparent shader:
You'll notice that there is some indirect lighting (in this case it's a slight orange tint to the shadow, because I made the diffuse shader orange)
If you don't want that orange tint, you can either:
Disable it per object by disabling Diffuse in Properties > Object > Ray visibility:
Disable it per material with the light path node: