I think I understand what he's saying. Basically the user is not logged into wordpress via any method. The "Allow FB Login to Comment (for non-registered users)" module is enabled.
Now on top of the comments form the user clicks the "Connect with Facebook" button. At this point the user is only asked to "Authenticate" with facebook. Allowing the website to get their Name/email/avatar. This is used for the comments form.
Now, the form reloads, they write their comments. They see the checkbox for "Share this with facebook" at this point if they check it, now they are prompted "Hey do you want this website to post to your wall."
And also, add a option in the administration screen to display or not display the "Share this with facebook" checkbox.