Oh, I didn't mean that the physical mounts have been mostly reverse engineered. I meant that the camera→lens communications have been fully to almost fully reverse engineered.

Mount optical characteristics are very simple. Just make sure your base mount has a larger diameter and a shorter flange distance, and you're golden. If you are designing your camera around that it is almost trivially done.

The code for implementing a basic E mount is on github, for the EF mount you just have to go looking around on some forums or buy it from the guy behind Metabones, same for Nikon G.