Graal only has 4 accessible layers for NPCs, and only ONE of those four layers draw under the players are ALL times(layer 0). This leaves very little room to be flexible with how things render in what order, especially for things on the ground like rugs and tables. The only way to decide the order of things on the same layer is extremely hackish.
I wouldn't be opposed to players be able to have a choice in the matter of how their NPCs layer though. But players will soon be annoyed in the same way scripters are with how limiting the layers are.