Thanks for your response, I was beginning to think this project is impossible. I've been in contact with a company called graphalloy which makes graphite bushings which can deal with high temperature. No water jacket would be required with a graphite bearing which could allow the shaft to be much shorter. The motor needs to be thermally isolated from the loop; it is likely that it will be connected to the shaft via a belt or chain, unlike the first diagram. In this case the motor will not support the shaft.
This is a more detailed idea for the shaft mount/bearing, my background is more physics than mechanical engineering though so I am not an expert in designing such things although it is an interesting thing to do. Any more advice welcome.