I'd be interested in hearing the rationale for changing the titles.
I actually prefer it this way. This way, HN is neutral - by presenting the article title as-is, they aren't asserting any opinions on the subject (usually).
When I put it that way, it only makes sense that the title should reflect the original article's 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.
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.