I'd have suspected that autosave is less of a database thing and more of an infrastructure thing. When you type something into your browser, it's all local - nothing is being sent to the server at all. You'd need a webpage which periodically pinged stuff out as well as somewhere to store it (and keep track of everything), or for it to be stored locally in the browser. I've no idea how hard such a thing is to implement.
One thing I have noticed though is that if you leave a page open for a long time, you can't post a reply (The old "Your submission could not be processed because you have logged in since the previous page was loaded"). Worse, it eats any reply to do try to post, and going back leaves you with an empty post box. However, the reply isn't lost - you can actually find it in the page source for the "couldn't post", it's just not visible.