While cleaning up my config and rolling back some changes, the error just happened to disappear. My nix-channel settings may have been a bit borked (mixing unstable / 22.11), and I also had caching/substitution turned off (although I was failing to turn it on again, nix.conf wasn’t updating for some reason, found some useful info in your answer here How to use binary cache in NixOS - #4 by NobbZ).
Finally getting my configs under version control now, sorry I didn’t have that earlier, then it would maybe had been easier to figure out the issue.
Still have the compressed and uncompressed versions of the patch file. What encoding are the hashes for patches in the .nix files (0f0d9iddg8zwy853phh7swikg4yzhxxv71fcag36f8gis0j5p998)? Looks like it could be base32hex, but it doesn’t match up with the files I got.
I’ll try turning caching off again and try rebuilding cogl later today to see if I can get a repro. Not sure if I can delete it from the nix store to force a rebuild if it’s still in use.
Thank you for your help so far @NobbZ!