The solution is to have a helper GUI process or shell extension (loadable DLL module for Explorer) which will communicate with the service and either add the mounting point itself or broadcast the message when the mounting point is added or deleted.
Why the added mounting point is not visible in Explorer?
Explorer will update information about existing mount points only after it receives the WM_DEVICECHANGE message. This message is broadcasted automatically by SolFS. However, if you are adding a mounting point from system service, the message can't be broadcasted: Windows doesn't allow messages to be sent between different sessions. And Explorer is run within different session from service processes.