I built a site like this a few years ago, got absolutely no traction, and shut it down.
I'm very open to the possibility that I went about it the wrong way, but I don't really know what I should have done differently. Maybe it could have worked if I had done a lot more evangelizing -- I admittedly didn't do much, but what I did do was so poorly received that I gave up.
I think something like this has to be integrated directly into github or the project's issue tracker to have a chance to gain any traction. But honestly, even then, I'm not sure it would work.
I'm very open to the possibility that I went about it the wrong way, but I don't really know what I should have done differently. Maybe it could have worked if I had done a lot more evangelizing -- I admittedly didn't do much, but what I did do was so poorly received that I gave up.