Hi Jack_O. Thanks for responding to the questions. It helps to put your design into perspective. I see you're not worried about solidifying anything, the pressure and temperature are to maintain your fluid as a liquid, so we don't have to worry about heating the bearing or seal area. Those areas can be relatively cool so the fluid will be subcooled liquid in that area. If that's the case, I think you want to run your bearings and seal at ambient temperature by providing a thermal stand off between the hot process liquid and seals/bearing area. All you need to do to provide this thermal isolation is to extend your shaft away from the hot portion enough to allow for a thermal gradient between the two. Heat is rejected all along this thermal stand off either by using fins to reject heat directly to atmosphere, using water as you show in one diagram, or some other method. The clearance between the shaft and housing in this case needs to be as small as possible to prevent the liquid from setting up convective currents between the cold end and hot end. In the picture below, you can see a typical high temperature pump that has the motor, seals and bearings at one end and the hot impeller a distance away on the long thermal stand off. I would think your Graphalloy bushings would be ok in the hot end but regular bearings and seals would be added to the cold end of the pump housing.
Regarding the use of bushings as seals, that might work but the problem will be in maintaining the two rotating faces perfectly parallel to each other. If one side lifts up or even if it just has a reduction in contact pressure, the seal could leak. The way it's normally done is to provide a pressure energized seal mounted on a bellows or similar device that allows the seal to follow the face it is sealing against such as in the picture below.
This image is for a seal on a boat shaft so it's very low pressure. The bellows especially isn't capable of handling any significant pressure. Better seals use metal bellows in this location, generally a type called "edge welded" to allow for the axial motion and to keep the seal against the hardened metal face. Spring load and pressure behind the seal help force the two parts together and create a seal. In your case, the flat washers you have might be able to accomplish this to some degree but they will need some preload, preferably by a spring pushing on the axis of the shaft, and they have to be absolutely flat and the shaft can't wobble at all or you'll have one edge lift up and it will leak.
Another option to your screw type pump would be a gear pump.
Again, you would still have a shaft that rotates inside a thermal stand off with seals and bushings at the end. Other options for off the shelf pumps might be found by searching ThomasNet
Rather than making your own pump (or purchasing one) to circulate the HTF as a hot fluid, you might consider adding a heat exchanger to cool it off, pump it as a liquid at ambient temperature, then heat it back up to put back into your process. The heat exchanger can actually have the hot fluid coming in to be cooled down exchanging heat with the cold fluid coming from the pump that has to be warmed up. This kind of counter flow heat exchanger is commonly used instead of operating the machine at high temperature.