So, the documentation is not comrehensive https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-registry.html#registry-format. But I've investigated the possibility of doing this a bit.

Doing something like e.g. this is the closest I've gotten:

{
  "flakes": [
    {
      "from": {
        "id": "rime",
        "type": "indirect"
      },
      "to": {
        "type": "tarball",
        "url": "http://rime.cx/http://rime.cx/v1/github/numtide/flake-utils/b/main.tar.gz"
      }
    }
  ],
  "version": 2
}

It might sadly be the case that there is just no easy way for us to do this in a way that supports arbitrary repositories. In that case, leaning on what we discussed on fedi, we might consider saving requests, allowing people to opt out with a query parameter like ?private=true, and having that populate a registry.json file.

I'm not itching to implement this yet, I'm hoping someone comes along and tells me there is a much easier, more elegant way to do this... 🤞