Not sure if that helps, but here is the code for the aggregate https://github.com/NixOS/nixpkgs/blob/76d197f4b03c0ad103b0254d6c333fca427133ba/pkgs/build-support/release/default.nix#L122

I am then doing something like that on my flake to define an overall hydraJob for all my nixosConfigurations

          ${name} = nixpkgs.legacyPackages.x86_64-linux.releaseTools.aggregate {
            inherit name;
            meta = {
              schedulingPriority = 10;
            };
            constituents = map (n: "nixos." + n) (nixpkgs.lib.attrNames self.nixosConfigurations);
          };