Ok a bit more thought and I think your answer is little bit of everyone's idea.

That is a fixed gantry design that a has a short Z axis mounted on on it say 50 70mm for machining operations but then use Jazz's suggestion of adjusting the height of the whole beam using a clamp/unclamping system to change to height depending on the work piece. The A frame idea could easily have this added as long as there is still a brace bar at the top.

This is not exactly revolutionary as this is the same principle that mills have been using for years!