Hi Nico,
Sorry, but either I did not understand your description of your problem, or I cannot reproduce it.
Here is what I did:
-created a teapot
-created a layered mtl
-left the base material with a default corona mtl
-created a new material in the next layer slot
-added CoronaAO as a mask for it
-nested a noise map in the CoronaAO's max distance slot
-it just works, no problems here...
If I did something wrong, can you write a step-by-step instruction? (1..., 2..., 3..., ...)