An extra DBus exposure that tells the running instance to check right now if a notification should be delivered according to the normal logic (and deliver it in the normal way if so).
If the app is not already running, dbus should start it and tell it to process this call. If that happens, delay loading the QML or showing any graphical element. Compute the notifications as normal, and if there are none just exit. If there is one, then load the QML but with the window hidden and issue the notification as normal.