I think you will have trouble using any bus with 500+ loads hanging off it.
How fast do you need to update the nodes?
Can you split them into a few groups?
What about daisy-chaining as that has no practical limit to the number of nodes (or the distance between them depending on the speed required).
Does it have to be a single motor per node? Can you say have 4 to reduce the number of nodes to 128?
Rob