Correct me if I'm wrong, but from what I understand, you want to fill an invisible region with mist (a bit like capturing a fire's smoke into a glass container).
This is not what this fog is about... Think of it as depth behind your screen. The only parameter influencing how much fog you can see at pixel X is how far from the camera the object drawed at pixel X is. The only thing the region does is trigger a change in the thickness of this fog for the whole screen.
(some game engines can create real "fog regions", but Plasma can't
)If you want to make an Ahnonay-like fog, you must fake it. You must put a planar mesh with a cloud alpha texture applied to it (a bit like you would create a water mesh).
Then, you must manually make the vertices under the fog whiter (this can be done with a texture or with vertex painting).
But it won't look as good, since the white-ness of an object won't be calculated depending of the distance to the camera...
Another solution would be to fake the fog (so that it looks like in Ahnonay's sphere seen from above), then use another Python script that would gradually increase the fog density as the avatar walks down into it. The mist would be applied to the whole screen, but it would look good enough.
Hope that clarifies things a bit...
