Greetings,
Metal o-rings are an option. I used them successfully for a high temperature Inconel heat exchanger. At 800F you are probably talking about silver coated Inconel or Inconel X. There are many items that must be considered in order to make a reliable seal (e.g., surface finish, optimal o-ring cross-sectional diameter, vented or non-vented o-ring, thermal expansion of flange and bolting, etc.). Metal o-ring are not forgiving like elastomeric o-rings.
Do a Google search on "metal o-rings" and contact manufacturers. They can help make sure you consider all relevant design considerations.