Not exactly what you wanted, but in Cube 2: Sauerbraten http://sauerbraten.org/ http://sauerbraten.org/ (open source) you can easily edit rooms in god view, and

Not exactly what you wanted, but in Cube 2: Sauerbraten http://sauerbraten.org/ http://sauerbraten.org/ (open source) you can easily edit rooms in god view, and then use the map for a first-person shooter.