it would be possible to avoid making many many many scenes for doing this
but it would require to bound buildings to entry points, and when the layer builds one of the new building, then the building in the scene would change its Z coordinates (it means every new building would be in the "original scene" but underground, and when the player builds one, it comes out on the ground, with new Z coordinates)
I think i mentionned it earier in this thread, but i really dont have any spare time to work on M&B right now. My master at univ. is taking way too much of my time.
If you're trully interested, i could give you some links and contacts of people who helped me trying to make this happen. I started it but didnt have time to continue this project. Here again, sorry!