I got my OLED yesterday and so far all three HDR games I’ve tested (Tetris Effect, THPS1&2 and modded Control) won’t allow HDR to be enabled. I cant see any workarounds for them so far either. Hopefully this will be improved with time, but as a start it seems HDR support is about as annoying as it is on desktop PC!
It was kind of a pain, but I figured out how to get Tetris Effect working in HDR on the OLED. I wrote how I did it here: https://www.reddit.com/r/SteamDeck/comments/17z25y5/which_hdr_games_actually_work_in_hdr_mode_some/kb8c8ea/