Bit Late..don't know if you got this figured out, but if you click on the scene in the hierachy tree then you can edit the "realistic sky" under atmosphere and there are moon settings there. I suppose you can also place a primitive that has been altered as a shader over that area. Playing w/ transparency channel. You could also do some work post process. Really depends on what you are going for.

No need to think outside the box....
    Just make it invisible.