I decided to write a an auto-registration mod for PHPBB3. It is very simple and easy to update. I am sure others can help make it better and more secure.
Add this at the end of /events/processors/join.php
define('IN_PHPBB', true);
$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : 'YOUR ROOT PATH HERE';
$phpEx = substr(strrchr(__FILE__, '.'), 1);
include($phpbb_root_path . 'common.' . $phpEx);
include($phpbb_root_path . 'includes/functions_user.' . $phpEx);
require($phpbb_root_path . 'includes/constants.' . $phpEx);
// Start session management
$user->session_begin();
$auth->acl($user->data);
$user->setup();
$user_row = array(
'username' => $_POST['user'],
'user_password' => $_POST['pass'],
'user_email' => $_POST['email2'],
'group_id' => 2,
'user_timezone' => 1,
'user_lang' => "en",
'user_type' => 0,
'user_actkey' => "",
'user_ip' => "",
'user_regdate' => time(),
'user_inactive_reason' => 0,
'user_inactive_time' => 0,
);
user_add($user_row, $cp_data);
Make sure errors are turned off in PHPBB3, or else it will halt the script for using POST.