Here's an interesting thought about medium gray. Some people say it should be 18% reflectance. Some say it should be 12.5%. How does this map onto the zone system? If we put it 18% reflectance at zone 5, then we get zone six at 36%, zone 7 at 72%, and zone 8 at 144% reflectance. This means that (assuming one limits the discussion to diffuse reflections), the most you can get is zone 7.5 in a scene, and that would be for a perfectly white diffuse reflector. Anything higher is a physical impossibility, that is unless one brings specular reflections into the scene.
If one picks 12.5% as medium gray then in the absence of specular reflections the most one can get in a scene is zone 8 unless one brings in the possibility of specular reflections.