Following email conversations with Eward yesterday, I have decided to leave the forum open to anyone to read, but posting still requires registration. I have set registration to require Administrator approval. That way if anybody tries to get in we'll know who they are and agree to accept them as members before they can participate.
The forum is still unadvertised and inaccessible except by entering a direct URL. I haven't put a link on the home page.
Do we all think that this is the way to go?