an "inofficial" way of doing could be this - and I say could be, as I did not test that. The terragen file can be edited with a text editor. So if you have an obj reader object inside your scene, you could try to create a sequence of this TG file. And in each TG file you could change the path to the refrenced obj.
But even if this works, this could result in heavy (GI) flickering