Submitting the 15K seems perfectly reasonable to me! I guess when I hear "rough draft," I automatically think of a completed (or very nearly completed) draft of the entire story.

But I don't have a problem with saying that only writers who actually have the 15K in hand already (and can prove it) can sign up for an artist. That seems fair to the artists, who don't know what they're jumping into. And it's good motivation to slower writers like me to make sure we keep on schedule.