You can vouch it, and if the comment is still [dead] but it's really good you can send an email to dang and tomhow hn@ycombinator.com Remember to include a link to the comment, and use it sparsely because it's a manual processes.

Like the parent commenter, I frequently see high quality posts via showdead. I vouch them, but I've never seen one resurrected soon after. I rarely remember to go back and check hours later, but by then the thread has died down anyway.