Sanborn references individualistic visual encoding systems in Kryptos. (ref: WIRED article, 2005)
Optical illusions don’t have to be that far off. I think there is some physical relationship between the letters carved into the copperplate and what you can see on the other side. Go check them out at http://www.voynich.net/Kryptos/. Keep in mind that Sanborn completely designed and executed that courtyard. It’s the trees, the landscaping, the orientation. He had complete control of where the copperplate would be, what direction it would face and what would be visible through it. I think there’s some place to stand to look through or time of year, time of day to see some shadow that will make it clear. You have to look through, look past the cipher to see what message he’s trying to send us. I think that’s the stego, that’s the visual encoding they both kept talking about. Think about Cyrillic Projector and MEDUSA…
I also think you just have to be there to see it. No picture will let us passively guess.