Here's a short-term workaround for Ver. 3.2.5 if you need it right away:
If security through obscurity is acceptable, you can make the following change to /www/bboard/index.tcl:
diff index.tcl index.tcl.dist
48d47
< and (bboard_topics.moderation_policy<>'private')
If you do this, any bboard of type 'private' will not be presented in the normal list of bboard topics. You can then create a link to the hidden board in a custom content section visible only to the group.