it sounds like you already know the [http://iPodlinux.org/wiki/Dock_Connector|iPod dock connector pinout] - I've heard that some accessories look for power provided by the iPod on pin 13 of the dock connector to detect it's presence. have you checked that out?
of course there's also the [http://iPodlinux.org/wiki/Apple_Accessory_Protocol|serial accessory protocol] used by some dock accessories, but I'm not sure if a radio/speaker dock would use that to detect simple presence of the iPod