Existing javascript magic (as in window.onPreUnload, see
http://www.hunlock.com/blogs/Mastering_The_Back_Button_With_Javascript) does not distinguish between the back button or any other means for leaving the site (e.g. submitting the form). So I don't see any way of implementing this currently. Changing the link to GET instead of POST wouldn't change the fact that you'd still use your edited content, so I don't see any point in this either.