I decided to try the same thing with art.
Here's my oDesk job post:
I asked applicants to sketch a nose. There was a bit of confusion about this, since my drawing doesn't include noses on the faces, so some applicants thought they were meant to sketch a nose on the face.
I hired the first three people that submitted a nose sketch, and I kept the first drawing they submitted — I didn't ask for any improvements. The whole process from job-post to final results happened this afternoon/evening. Here's each person's application sketch, and their final drawing: