I just implemented this for a client (in ACS Classic :). There used to be support for Private bboards, which required a password for access, but that has been mostly ripped out even though you can still choose Private for the moderation style.
I won't offer to make code changes available yet, because the client claims he can get into a private bboard with an unauthorized user. I can't duplicate his results, but clearly something is still not right. In concept, what I did was this:
I added a new user group type called Private Topic. Then when a new bboard topic is being created, if it's moderation policy is set to private, a Private Topic group is created for it instead of an Administration group. This way only members of that group are able to see the topic.
Oh, this also requires fixing a bug where the group is created for the topic, but the group_id is never saved! In a way this bug is a blessing, though, because they put code in place to restrict viewers of the topic to the group, and you don't really want that if the group is an Administrative one as is created by default...
I'll try to remember to come back here and let you know when I've got it right. Roberto, feel free to e-mail to remind me if I seem to have forgotten :).