That's pretty cool. I was thinking of bundling a default.nix in the root of my projects with maybe a recipes/ folder for things like ruby, libxml2, ...

It's definitely possible to do that and even link to nixpkgs but then your projects dependencies might break each time you updated nixpkgs. I don't know of a nice way to solve that.

My sample looked something like that:

    let
      pkgs = import <nixpkgs> {};
      stdenv = pkgs.stdenv;
    in
    stdenv.mkDerivation {
      name = "my_project";
      src = ./.;
      ...

There's also a deps.mix for just the dependencies. It's on an internal project so I can't really divulge the details but email me if you want to talk more about it.