@rycee s home-manager is distributed over a channel.
The GitHub repository lives at GitHub - nix-community/home-manager: Manage a user environment using Nix [maintainer=@rycee].
It contains a default.nix at its root. As GitHub provides a tarball for each commit, as well as the “tip” of each branch, and each tag, you can use that tarball as the channel URL.
It as far as I remember the pattern for the name is https://github.com/${user}/${project}/archive/${ref}.tar.gz, where ${ref} is commit SHA, branch or tag.