I actually have that in another demo that I made...
it works with bridges... I can give you that if you want ^_^
Another idea, if you are having multi level maps (with lots of levels) to have a lot of the same map, just with different settings (ie you can walk over a bridge on one map, then you duplicate the map, and make it so you walk under it in another map) and then when you are walking on stairs up and down the level, use the "transfer player" command, and turn off fading.
it works, trust me ^_^