Да, вы можете и вот грубый пример:
$this->load->library('form_validation');
$this->form_validation->set_rules('username', 'lang:lang_username', 'required|callback_check_login');
Тогда для обратного вызова:
public function check_login($username)
{
$username = $this->input->post('username');
$password = $this->input->post('password');
$remember = $this->input->post('remember');
$login = $this->users_auth->login($username, $password, $remember);
if ($login !== TRUE)
{
$this->form_validation->set_message('check_login', $login);
return FALSE;
}
else
{
return TRUE;
}
}