Thanks, Mark, but  it's scale that I need, not translation.  Regardless of what manipulator I use, I can only scale in the direction of one of the faces of the working box, which moves with the object, akin to Bryce's Object Space.  What I want is a function similar to Bryce's World Space, where the bounding box would remain parallel to the ground plane regardless of the orientation of the object.

That way you can use scaling to twist a terrain to create overhangs, then rotate it back to its original position.

The only way I've found is to convert a terrain into a vertex object, but you lose all the terrain functions---a major detriment.

Any ideas?