I see a bit of a conflict between

    Does CodeMill work on private repositories as well?
    Yes, the only difference is that interested developers won't be able to fork them unless you authorize them...

    How do I screen developers?
    That's part of the beauty of CodeMill -- It's based completely on GitHub, where each developer has their public profile

This might be useful for orgs sponsoring open source projects but for orgs trying to get cheap labour on internal projects, there's no way of appraising previous similar work.

(Also, I have only ever heard the word 'mill' is used in a pejorative context: "Diploma mill", "essay mill", etc)