Sure, here is a picture of pretty much what I'm looking to do:
Hoping "fog" can be extended to other "weather" style effects too, like rain/snow.
I'm using layers to significantly help with z-ordering... as it stands, the way I'd like to implement fog, I couldn't have everything on the same layer and use simple z-ordering anyway, since the fog would have to be both in-front of things AND behind things at the same time.
As pictured, the "structure" has to be behind the "actors", but in front of the fog - and the "actors" have to be in front of the "structure".
The only way I can think of to do this, is to have the fog opacity set to "0" if overlapping the structure, but this would create a "soft finish" around the structure (unless my fog sprites/particles were very small and in great numbers) - which is not ideal for performance.
I was hoping for a mask of some sort to define where the fog would be omitted on the layer set up for fog (which would preferably be in front of everything else). I'd like to make this layer a "weather" layer ideally... so I can do fog/rain/snow etc.
Perhaps there's a better way to do this entirely? I'm not sure!