I love well done graffiti, especially if it not only looks good but it is in an appropriate area (those areas that really are ugly without it and not something that looked nice beforehand)... but I do feel this is really REALLY sketchy ground.
Found graffiti only is a very idealistic rule, how do you enforce this? How do you know if it is found or made? Even if you do find an artist who is somewhat honest and won't submit it himself, he could still get his friend.
Even with this problem, you still will be sifting through a ton of artwork. Everyone thinks they are a great artist and everyone has a different idea of what good grafitti might be so i imagine you'll get a good amount of bad graffiti.
I wish you luck if you get it. I think if you do I would also add the restriction of appropriate place. I don't care how good the graffiti is, if it is on the side of the Lincoln Memorial or something, that just isn't right.