The reason the camera controls the exported geometry area for the micro exporter is that the terrain geometry is created by the rendering process and the rendered area is set by the camera view.
If you want to export a square region of terrain, you can turn it into a heightfield by feeding your shader network into the Shader input of a Heightfield Generate, then use a Heightfield Export LWO to get some geometry out. But the micro exporter is generally superior, being able to output in OBJ format for one thing, and you can achieve a similar effect using a top-down render.
Limiting geometry export by a mask is not currently possible.
- Oshyan