[_Microft]

"Keyhole Imaging" is only a catchy name for the technique. It works with any other opening, like a door crack for example, as long as you can shine a laser through it. I would expect that it is even possible to use an opening that is covered by a light diffusing material like paper or tape on the inside. Instead of measuring reflections from the far wall, light diffused by this material would be used instead.