My only suggestion would be to call a local fuel and oil supplier. Chevron et. al. supply a lot of different petroleum products and would have some recommendations.
The most water- resistant grease that I know of is Dow Corning High Vacuum grease, but it's not good for 1000F.
The trick with exhaust systems is to make the supports long enough and slender enough so they don't conduct a lot of heat, and radiate most of what they do conduct.
