I have been working with exported heightfields in Blender to make image maps and locate objects. I have not been that successful yet. But it is not that hard using Blender to group objects/models and export them as one large object.
The problem is that Blender units do not scale. They have to be re-scaled in Terragen 2 to the right size. More often than not, the re-scaling results do not fit the heightfield properly. It will work if an object imported into Terragen 2 is part of a larger, say building, which share a common center point.