Hi Gene,
I misunderstood your point. It is slightly confusing. When the thread is actively locked by the admin, you cannot respond to it, AND you will not see a field to post in. If it is not locked, you will see a window to reply in, but you will encounter a message saying, "Back off; you cannot respond to an announcement. The original poster can, because the original poster must be an admin or be a moderator with those rights. Since you are a moderator, I could give you or a group rights.
But, I am probably wrong about the latter, and Ruben can correct this. Other than being able to say "psyche" I don't see much of a difference between announcement and locked and announcement, other than the locked option is just available for all posts by those with rights.