If anybody wants to see this in action, I've installed it on a live site at
Cool Sci-Fi. Just click on any of the threads as a guest (eg:
random thread). At the bottom of the screen you'll see the quick reply editor as if you were logged in. Enter a reply and hit the 'Post Reply' button. As soon as you do a pop-up overlay will be shown to you where you can register an account.
If you install it on your site, after uploading the files to your server and importing the XML file you will first need to go into your Options to explicitly specify which forums will allow quick reply for guests. It is a per forum opt-in choice, not opt-out. You won't see the quick reply box until you specify the forums.
For users who are already registered then if they do a quick reply as a guest and enter their credentials on the overlay then the post will be submitted seamlessly.