BuddyPress.org

The Group Activity Subscription plugin has an option to send a notice to all group members regardless of their notification settings. You might use this in combination with the BP Group Management plugin. You could use that plugin to add all members to an “Announcements” group. Just make sure you keep on top of it so that everyone is always a member of the “Announcements” group. People can still leave the group… so this isn’t great. But it’s an idea anyway.