I'm curious about this too.
I would have thought data-duplication with usernames and passwords would be a hazardous situation?
There must be a better alternative. The only thing really would be to have your custom registration process add people to the PHPBB users table, and share that. the problem I found (and this is going back a bit now) was to do with PHPBB's password encryption, and getting around that. Can you configure PHPBB to not encrypt user password?