As far as I know what you're trying to do does not exist in Maya. However, this is a total hack but it could work.


Try making some layered shaders with the shaders you're using as the base layer, and a new shader above them that uses a projection map to blast transparancy accross the terrain. Then you could move the projection node up and down to render each level.

OK, so my description sucks, but I think it should work. Let me know if you need more help and if I have time I'll put together an example.