That's a brilliant idea! .... Years ago I had thought of doing something like that to a Pinzgauer chassis, because some friends were talking about improving axle articulation on factory vehicles.
It's relatively easy to add long travel suspension components to a standard chassis design like a LR or Tojo, but far more difficult to do it on a backbone chassis design, so "flexing" the central tube was in our opinion the best and most viable option!
The COOT 4x4 (and others) used a pivoting backbone to allow for axial articulation. It is a very simple design.
My biggest question is how they solved the shift lingage issue. Rigid rod like the original is obviously out. A stiff cable connection from shift lever to transmission would be the best bet in order to allow for both forward and aft shift rod movement and rotation motion as well.