One possible way, depending on how your scene is set up, is to use the render passes.and compositing
The Diffuse Filter passes (there's two, either should work [even if they differ slightly in my test]) gives the objects diffuse colors, without any shading. You could use that, mask it out with info from either the Material Index pass or the Object Index Pass, and composite over the rest of the scene.
This doesn't work if it's behind glass though..
Windows 10 Pro - AMD Ryzen 7 2100X 8 core 3.70GHz - 32GB RAM - GeForce GTX 1080 8GB Octane Prime - Blender Plugin user