I've tried that before. I was able to get my orbit down to, on average, 4km. I had to do a lot of looking at ISA to find the lowest route through the trans-polar mountain range. A polar orbit wont work, the poles are just to mountains. The Mün is pretty flat, it is really just the trans-polar mountain range i mentioned. My orbit had a slight eccentricity and was only able to complete 4 orbits at that altitude before it crashed into the side of a mountain. So it is possible to get a really low orbit, but it isn't sustainable and it is a grueling process. I spent about 6 hours game play and probably something like 10 hours with planning included. Remember, you cant time warp at that altitude.