Thanks for the post Falco.
Not PMs tho, they are person (or group) specific and unfortunately off the main boards (so don’t show in latest lists etc). (Plus I’m not sure you could even do a group PM for all members and then every new member that joins afterwards?).
The feature would be handy for sporadic threads which we just want to restrict to members-only (but do not want put in any special section). They could have the same icon that appears when a topic is unlisted or in a private section.