I have the code, which involves a fair number of files, so it makes no sense to post them all here.  In general, the concept is:

1. authentiacate the user with facebok connect (i.e. button -> launch page [probably with javascript] for user to accept allowing your app)

2. upon successful login to facebook, create a phpbb user programatically

3. login automatically/programatically with the newly created user

4. change viewforum.php/viewtopic.php so that it maps the phpbb username to the facebook name (first name, or some combination of first name and last name).

5. disallow people these phpbb users created via facebook from going into ucp.

6. disallow showing of captcha if user_login_attempts >= max_login_attempts in includes/auth/auth_db.php, i.e. the line:

if ($config[‘max_login_attempts’] && $row[‘user_login_attempts’] >= $config[‘max_login_attempts’]) …

It takes a bit of reverse engineering.  Other than that, it was alright.