It would just be a small code on the "View Comments page" right under the "Post a new comment" link on the comments page.
This could be helpful for allowing a quicker reply to comments. Also it would cut down on the load for LJ because you will not need to load another page...therefore making the site faster...in theory.
It could only be visable if you are logged in and just has a Subject line and the Reply box with a button to "Send Comment." Nothing else. Basic. Keep it simple.