I was able to hack in a fix by editing one line in class-fg-drupal-to-wp-admin.php, line 495.
case 'mysql':
// MySQL
$drupal_db = new PDO('mysql:unix_socket=' . $this->plugin_options['hostname'] . ';port=' . $this->plugin_options['port'] . ';dbname=' . $this->plugin_options['database'], $this->plugin_options['username'], $this->plugin_options['password'], array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES \'UTF8\''));
break;
I changed the host param to unix_socket, I’m then able to type in /my/path/to/my/mysql/socket for the hostname in the UI and connect.