Not sure if anyone has done it, but maybe start by looking at the way it pulls wordpress users from another database?
https://bbpress.org/documentation/integration-with-wordpress/
Trent
There are tricky new possibilities within the trunk (build 1075) that I have not researched yet. Using extra definitions in your bb-config.php it allows you to have separate tables, even separate DB’s for users. It might allow you to do that.