So jorn has commited my patch that implements the magic of autoconnecting action and state-variable query signals to GUPnPService. I am too lazy to write all about it so I'll just copy&paste the doc-comment:

/**
 * gupnp_service_signals_autoconnect
 * @service: A #GUPnPService
 * @user_data: the data to pass to each of the callbacks
 * @error: return location for a GError, or NULL
 *
 * A convenience function that attempts to connect all
 * possible "action-invoked" and "query-variable" signals to
 * appropriate callbacks for the service @service. It uses
 * service introspection and GModule's introspective
 * features. It is very simillar to
 * glade_xml_signal_autoconnect except that it attempts to
 * guess the names of the signal handlers on its own.
 *
 * For this function to do its magic, the application must
 * name the callback functions for "action-invoked" signals
 * by striping the CamelCase off the action names and either
 * prepend "on_" or append "_cb" to them. Same goes for
 * "query-variable" signals, except that "query_" should be
 * prepended to the variable name. For example, callback
 * function for "GetSystemUpdateID" action should be either
 * named as "get_system_update_id_cb" or
 * "on_get_system_update_id" and callback function for the
 * query of "SystemUpdateID" state variable should be named
 * "query_system_update_id_cb" or
 * "on_query_system_update_id".
 *
 * Note that this function will not work correctly if
 * GModule is not supported on the platform or introspection
 * is not available for service @service.
 *
 * WARNING: This function can not and therefore does not
 * guarantee that the resulting signal connections will be
 * correct as it depends heavily on a particular naming
 * schemes described above.
 **/