If your SME server is server-only, then your outside "webmasters" must gain access somehow to the local network, yes? And unless you're committing the security error of exposing your SME Server's LAN interface to the Internet at large -- thereby defeating its built-in security features -- then your outside users MUST do something such as RAS, BorderManager, PPtP or some other form of authentication to gain access to the local network. Right?
Once inside, they can access the server manager and/or use a client such as WinSCP or PuTTY via the SME server's LAN IP.