If you're going to stay in the confines of bboard, you might want to allow posts (referenced to user_id = 2, anonymous) with another field where they can specify their emails without logging in. (similar to slashdot circa fall '98 [or old bboard?])
Shouldn't be too much code you need to change, but I haven't looked at the bboard internals.
If you anticipate these users coming back and signing up, you can then "update bboard set user_id = :new.user_id where email = :new.email or from_ip = :new.from_ip" or something to gently bring people into the system and preserve their earlier work.
Anyhoo, have fun.
Something I've looked at earlier, but generated no code, is auto-creating users