I think the reason writing in the forum style is exciting is because having an audience for anything you do is exciting. Also, you can teach, moralize, scandalize, etc and get near-realtime reactions to the thread being teased in the writings.
I think the risk would be that if a writer edited too heavily based on feedback, the work ceases to be purely the writer's and now it's a collaborative effort, which may lead to an impersonal writing style or threaten one's claim to copyright.
In my opinion, the best place for an app like this is within Reddit. You have the willing audience already in-place to offer you the feedback. Although, I could imagine an unfiltered forum might present a distraction. There might need to be a separate forum moderator that can control visibility of comments to the writer.
And then when they're done they could export it to a text editor of their choice, or save it to dropbox or something.
I once made a chat app in Flash a long time ago called Mimic, that would take in whatever you said, infer connections between words (based entirely on what words were next to other words, so you could use just about any language or slang you wanted), and then build a sentence off that in response, that would almost kind of make sense. I would a good hour or more at a time responding to that thing trying to train it and getting a kick at the silly things it said.
This would be kind of like that. Yes, it'd be a pretty dumb app, but I could see myself being entertained by it, so maybe it'd be entertaining to other people as well.