Good idea.
Actually, we can stack them a little using system D : conveyor pole stackables and the fact we can put merger/splitter on conveyors, but it's not perfect at all (no snapping and the collision issue with splitter/merger once they're in place doesn't help: we have to remove them each time something is wrong).
I think a temporary way (before a total "stackability") to stack them would be, when you put a splitter/merger on a conveyor, to snap it when it is close to a conveyor pole stackable.