Are you sure ?
Don't have much time to complete a fairly difficult test is not easy.
Some (most ?) developers do not work great under much stress.
Most of the time the success in a code challenge is about having already solved something similar.
Sure but you can get much more from this simple experiment.
You can answer question like:
And it is completely risk free, just send me an email and I will add you to the companies page.
Sure, you can suggest those project, however if a candidate made a wonderful contribution to some other project I am expecting you to treat her as well as all the other candidate.
As soon as some candidate make a pull request I will scan through such request pretty quickly just to make sure that is not completely garbage, if the pull request is acceptable in quality I will introduce to you the developer.
For now it is completely free of charge. Enjoy ;-)