:
Ah, so it's possible to make background Trap Door in AE 
Edit: I usually think maybe too fast and then do instantly... i must start check closer
Now i have to guess "byte"s =/
Sprite hiding trick, scaling, etc  
Thanks for lighting... =)
Edit2: Copied background trap door and standing on it...
Now only Sprite trick and image editing is needed =)
 |
What about just putting the rect "off" the screen so you can't see it? AE might not allow you to offset the sprite.
Also its still going to be a while till the next editor version, got tons of stuff to sort out with CAMs and not much time to do it!