I messed around with the map of the Divide, and I noticed a clear winding path from the canyon wreckage (which will have us enter from the east) to the hexagon at the top of the map, which I believe is where the battle with Ulysses takes place. Take a look, I think I might have the legit path. I highlighted edges in green, and the path in red.

My Map: http://s1224.photobucket.com/albums/ee368/10303816/?action=view¤t=701px-Nvdlc03dividepleximap.png
Regular Divide Map: http://fallout.wikia.com/index.php?title=Divide&image=Nvdlc03dividepleximap-png

I would assume that the white dotted line that hella stands out would be the entrance from the Canyon Wreckage.

Looks like it. It's right between 2 mountains like the canyon wreckage.