We have had a couple of reader-directed stories posted here, and we do allow them, within certain parameters.
The voting and discussion does need to take place here on the forums, but if you use the threads where we have review replies, guests can read and respond to your post. This gives everyone a chance to vote and discuss the story.
The basic premise of the story can be treated as a prologue, I believe, which would make it story content. An author's note of no more than 600 words explaining how this works, with the url for the forum thread, is fine.
We don't have a specific tag for a reader-directed story, but we do ask that you tag for what content you think will be included, and if anything unexpected comes up, you can tag that specific chapter with the additional warnings as an author's note.
In short, we'll work with you to make this story work for you and the readers, within our ToS. We're good at that.