I never found a way on my laptop to roam within a hidden SSID net - short of attaching to a new AP as the old gets completely inaccessible. It does not seem to be possible while maintaining a connection, as nothing short of promiscuous sniffing or a connection attempt (which both severe the existing connection) would tell you the SSID of a hidden AP. If any, you'd have to assume being entirely within range of your hidden network, and hop channels blindly - but that will cause a lost connection whenever you hit upon a foreign hidden net.