config.php can't be encrypted directly using zend_guard. By encrypted it by mcrypt fonctions, it can be done, if there is a key in a crypted file. But, the php bitcode, is available to third party after it's decrypt and send to the php execution engine. So, if you have access to the file, just modify the php.ini and use a bitcode decoder.
Yes, it's more secure, will create some difficulty for the majority of pirates. But any good hackers, it's not a big problem. Using a server licence, the attacker will need to run the decoder on the server, and not offline.