SpudFiles

I don't know a lot about them myself, but there are different materials for orings, perhaps there is some exotic material that would stand up to the heat? Still probably need cooling to stay within their operating range.

Again, not something I know a lot about, but car engines use metal rings on the pistons don't they, maybe that's an option?

The only O-ring that I know of that is made for high temp is silicon. The McMaster site should be able to help you out there. There are others that are made for steam, etc., but I don't know what temps your system will reach, keeping the cooling in mind.

The rings on a car engine would possibly pose a problem with too much metal to metal erosion of the cylinder, unless your cylinder is going to be fairly hard. A Moly alloy would be very beneficial in this case.