Well,
it seems to work if I modify the terrainaltitude in the sbx: The terrain looks larger in the editor (though ceiling and altitude are still at 1000), and the value is preserved even if I save the project again. Of course, I still have to see if it really works.
Regarding roads - by looking at the Lua script, I think I might come up with a solution that fits my needs. Is the binary data in the sbx just the roadmesh, serialized by the call to Save(stream)?
I have a different road question, though: If I use the "Align terrain to roads" button in the toolbar, my road gets very bumpy. I was expecting the terrain heightmap to be modified, but instead it seems the road aligns to the terrain... Did I misinterpret something?