The nav mesh isn’t captured and unfortunately I wouldn’t know a fast way to do it. if you use terrain what you could do is temporarily replace the terrain’s material with a solid green material and make the snapshot. A feature that is included to help you generate a good image to draw over is the ability to hide actor classes and specific level actors in the generated background texture.
Generating a texture of the nav mesh is an interesting idea. I’ll think about this some more and experiment with whether this is possible!