Nice idea on the chain but my original thought was to kill 2 birds with one chain so to speak in that the pulley running on the belt would effectively act as a bearing. You would still neeed another lower down to give some rigidity but the 2 belts per side increases the complexity and I was after simplicity mind you I realise that simplicity and accuracy dont go hand in hand.
Mind you ball screws are just not cheap enough.
Peter