A previous post suggested the same thing on an lcd display, which could also display live images from cameras which would show the ground below.

It's really too much trouble to put it in the window when it would work just as well somewhere else that wasn't structural. Also, these things break--it's far less hassel to snap-out a broken screen from a seat-back than it would be to get a certified airframe technician to come remove it from the window.

Otherwise, you've got a great, fun idea. But as with many ideas, I don't understand how someone could make money with this, so I don't know who would sell it or install it.

It could be offered by the airline as an option, like renting movie headphones.Participating customers could be issued a portable display, which would plug into the armrest, or be wireless, IR, etc. Arrangements might need to be made before the customers boarded.

The same display might be used for movies, in-flight tv, etc.

If you're talking a simple literal overlay, shouldn't be a problem. But if you're talking about synchronizing path/location data with what's below (kinda like using the classic "Hybrid" mode), you'd need more equipment. Without eye-tracking capability, there's no way to accurately do this—helicopter pilots wear special helmets. You could compromise and offer a limited set of devices (like how there aren't individual bathrooms) with some view-constraining system.