John,
Sorry maybe I mis-read the situation here, are you using external nameservers to resolve your current setup? If you are, then the information I gave you should work as these settings will only effect the internal network and only if the SME server is set as dns server for the internal network. The system will use it's own DNS first to resolve an internal request if it can't resolve it it tries a list of public dns servers. I doubt if you are using your SME as public DNS seeing you only mentioned opening port 80.
In other words to resolve names your external nameservers are first choice for external users, for internal requests SME has first go at name resolution and if it has an entry for that request that's what you get, despite your external name servers.
Regards
Jack
PS depending on your dhcp configuration, serv er-manager may require the mac address of the server and there should no need to publish globally.