After having played around with day/night cycles and the atmospheric horizon instead of a skybox for some time, I have to say that this, combined with volumetric clouds and a color change routine, looks fantastic... if not for one small detail that is off: The missing "horizon".

In Skyboxes often there is some far away scenery visible on the side panels besides the sky itself. To get this into the "dynamic" sky system, I propose some kind of Skybox with an Alpha channel (being transparent where the sky is), which is drawn after the sky, atmospheric horizon and celestial objects.
Ideally the opaque parts of this cubemap would also be taken into account for the shadowmap for celestial objects.

This cubemap would solve two problems present with the current dynamic sky system:
- giving the scene a distant horizon that interacts correctly with celestial objects
- lower the chance of ugly shadowing bugs caused by backface culled terrain, eliminating the need for either dropping the backface culling (bad for performance) or suddenly switching off celestial objects during dusk (might lead to a fake looking sunset)

I don't know if there are better or easier solutions for this, I could find / think of any.