Nix in single user mode can apparently work with SELinux in enforcing mode, although AFAIK binaries installed via Nix can not have SELinux metadata which could be an issue for some programs.
Determinate Nix claims to have seamless integration with SELinux (unlike upstream Nix and Lix, but it's not a fork, alright whatever you say Eelco). Using that and removing the proprietary garbage their installer also adds might be easier than making regular Nix play nice with it.