So, I found some forum posts, but all the apps in them aren’t updated anymore or don’t work. The thing on the wiki needs home manager (which I don’t want to use) and it doesn’t work with locally installed cursor themes. In the same entry on the wiki, it also recommends xsession.pointercursor, which doesn’t exist.
If you don’t want to use Home Manager, I’d imagine that you basically need to globally setup the global configuration files that correspond to each of these per-user configuration files : home-manager/modules/config/home-cursor.nix at 208df2e558b73b6a1f0faec98493cb59a25f62ba · nix-community/home-manager · GitHub
I couldn’t quickly find a corresponding global nixos option, but I may have missed it.