Currently it's a GNU/Linux package manager, but there's a working GNU Hurd port that is being finished up, and I don't see a reason why someone couldn't port it

Currently it's a GNU/Linux package manager, but there's a working GNU Hurd port that is being finished up, and I don't see a reason why someone couldn't port it to one of the BSDs if they wanted to put in the effort. Nix, the project Guix is based on, has been ported to OS X and there's even an experimental Windows port, so its possible to support many systems.