I note that not a single mention has been made about ingame editor mode/co-op editing.
I mean it's one of the main selling points of the Cube 2 engine. If I remember correctly (and I may not, I only played this back in... I think 2005 or 06) just press E and you can fly around in freeview, edit the map etc. in realtime. It's easy enough to do too, although I don't remember much about the controls.
I'll probably download this tonight and have another play with it.