Larry,
We already have a very similar notice available,
Locked Group,
so I would suggest to ask adding an
Announcement-Only Group
notice as a #suggestion in beta, so if one posts to an
Announcement-Only group they'll get the normal message they get now
but would also get the notice which would then free you from
sending an explanation email yourself, it'll be automatic. Since the
infrastructure is already there, adding the notice should only be a
matter of adding it in the database and adding a small amount of
checking code in the same place where it checks for the above Locked
Group condition and emails that notice.
Cheers,
Christos