If you're creating this in GIMP or Photoshop, it would be easy to just keep Elyon hidden by "blocking out" that fog on a new top layer; that way you'd know what's in there but no one else would. Just an idea--or maybe that's what you planned all along?
That was what I am planning. I am just not sure how to do it artistically. I think what I might do is to find a cloudlike pattern, and then color in everything beyond the coastline using a new top layer.

I will also be doing a map for Elyon itself, but the people of Gibeon are not aware of what is there.