Reprocussions of Master Selection

I have played around with it and looked through the code but I was curious what the implications of picking Drupal and PHPBB as the master registration system is? I haven't really been able to find a difference thus far besides the fields required for registration. I am curious because we also integrate some CiviCRM legs into our site (slightly modified) that act as a slave to the drupal registration system. So would setting PHPBB as the master system cause problems with that link or would it be more along the lines of A -> B -> C so A -> C (PHPBB indirectly passes to CiviCRM through drupal). I ask because I am curious about possibly using someone logged into phpbb through the embedded phpbbforum integration automatically submitting their information to civicrm modules when they click them (such as an event registration ect). I know the reverse is possible through simple sql lookup (posting the events registered for on phpbb) if the first is possible.

I know it is a very specific question to my circumstances, but I figured being the author, you would be able to give a clearer answer of how it actually works then I could just combing through the code.

Thanks!