This is really great, using the information available so far I managed to make a test track (I call it a "skid pad", with a jungle of trees surrounding the road:

Visible

The coordinates described here work perfectly for me, what's most puzzling is the "unknown" bits of the structure. Problem I'm experiencing is, at one point on the track, lots of trees are visible but most of them will disappear as soon as I move just a little bit (1 or 2 meters away from the "last good" point. The trees are still visible from the mirror though, see picture below).

I guess the unknown bits in the structure most likely have to do with the visibility managed by the MM2 rendering engine. Would be real nice to have this stuff sorted out.