Good question. NUR doesn't seem to output the packages directly, but requires that you supply your pkgs manually. You can use nix shell with NUR, but it gets rather ugly:

nix shell --impure --expr '(import (builtins.getFlake github:nix-community/NUR) { pkgs = (import  {}).pkgs; }).repos.mic92.hello-nur'

nix registry doesn't help here, as it's just for managing aliases that allow you to type "nixpkgs" instead of "github:NixOS/nixpkgs/nixos-23.05" in some places.

It might be possible to write a flake that outputs NUR packages for direct use, e.g. something like:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
    flake-utils.url = "github:numtide/flake-utils";
    nur_src.url = "github:nix-community/NUR";
  };

  outputs = { self, nixpkgs, flake-utils, nur_src }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
        nurpkgs = import "${nur_src}/default.nix" { pkgs = pkgs; nurpkgs = pkgs; };
      in {
        packages = nurpkgs.repos.mic92; // FIXME: this needs more work and filtering
      }
    );
}

Which gives:

$ nix --allow-import-from-derivation flake show .
git+file:///
└───packages
    ├───aarch64-darwin
    │   ├───bing-image-creator: package 'python3.10-bing-image-creator-0.4.4'
    │   ├───clearsans: package 'clearsans-1.00'
    │   ├───cntr: package 'cntr-1.5.1'
    │   ├───conky-symbols: package 'ConkySymbols'
    │   ├───eapol_test: package 'eapol_test-2.10'
    │   ├───edge-gpt: package 'python3.10-edge-gpt-0.13.1'
    │   ├───fira-code-pro-nerdfonts: package 'nerdfonts-3.0.1'
    │   ├───gatttool: package 'bluez-5.66'
    │   ├───gdb-dashboard: package 'gdb-dashboard-0.11.4'

But needs some more work and might be reinventing the wheel. Haven't used NUR myself and no idea what the state of flakes in NUR is.