True, but a paid short-term contract to do an actual task for the company would work. Knock out a small feature during the night/weekend. Keep it something contained and relatively straight-forward. If they can get it done, it demonstrates the candidate's ability to parse your codebase (so they are learning something they'd need for the job anyway) and they have contributed.

I think most companies just don't want to take the time to set this kind of thing up. Easier to just pass on people that won't do the requisite song and dance.