nix builds are now available in updates-testing repos.
There is also an initial test package of nix-2.24 for epel10 now in my copr repo if anyone wants to try that.
edit: epel9 looks tricky (RHEL9 seems to have sqlite version incompatible with nix-2.12 at runtime). If someone can recommend an older working nix version I can try with that (maybe worst case could be 2.3, which I think is considered insecure by now).
Just out of interest, are there any plans on dealing with the SELinux? From my experience it was the major issue for nixpkg CentOS-based OSs Or are you going with the single-user mode in this case, as it is the least painful way?