The thing is that the attention a submission gets is almost completely determined by the title.

A crappy title will ruin the chances of most people even bothering to look at the comments.

So I don't think this is a workable idea. Just leave the submitted title as is, and let the HN readers vote it up if they think it's worth it.

I wonder whether a simple post-downvote would be more effective than the existing flagging mechanism

This is not reddit. Stop worrying about such things.

I look forward to some social news site offering some version of this; it could help create more informative and time-saving titles. Alas, I don't expect it here, given the HN minimalism.

Roughly how it might work: in addition to the 'comments' page, there'd be a 'headline' page. (Borrowing terminology from journalism, this might also be called the 'hed' page.) Posts here would be proposed alternate titles; the most upvoted would be the default on lists (like top stories).

Perhaps, list views could show the top 2-3 titles (maybe as a user-specific option, or only when there's controversy).

Each submission could also get a 'subhead' ('dek' in journalist lingo), for more exposition/teasing of content. It'd be voted on the same way.

Perhaps, even, a similar approach could be used for competitive summaries, but I suspect that's likely to result in more controversy and less consensus/convergence.