I'm looking to do an experiment involving hot chlorine gas (900 Celsius), however at the moment the only material I'm aware of that could cope with this is Hastelloy's C-22 alloy. What other alternatives exist?
Short answer — none. Hastelloy C276 or C22 are the best alloys, also considering availability, presumably cost is no object. At that high temperature there is no readily available data. At 500 °C for a limited time experiment, other nickel alloys like 625 or 201 and even 316 SS have some limited life. But for 900 °C, I expect the Hastelloy C family was recommended because they have the best life at about 500 °C. Your conditions are where it would be best to get a manufacturer of high alloys or ceramics to work with you.