So don't propose it?

You can run Nix and Home Manager on macOS or any Linux distro. You don't even need root.

It works exactly like you describe: it provides its guarantees for all software that you manage through Nix, and doesn't get in the way of software that you don't manage through Nix.

Unlike with NixOS, you can run binaries that expect an FHS-compliant system just fine.

You can just silently use it and enjoy the convenience for the declarative parts of your setup, with no detriment to your ability to run the imperative, ad-hoc setup scripts that your company requires.