Over the weekend i wrote a very crude mod that will allow Roster to sync a phpBB group.
You basically specify a group number to keep synced. Click the button on Roster and it will remove all members from that group then try to match usernames from phpBB and WoW ingame names. If a match is found then they are placed back in the group.
(Obviously it requires that phpBB users have the same username as there in-game name)
Seems to work well for me. As yet i havnt wrote anything about ranks, i may do in the future. So that a "Officer" and a "Trialist" can go in seperate groups.
Let me know if your interested, i wont bother submitting it if no-one cares.