It seems the ActionInvoked and NotificationClosed signal isn't exposed anywhere.
The signals are defined here:
https://github.com/KDE/plasma-workspace/blob/master/ dataengines/notifications/ notificationsengine.h#L84
And the signals are called here:
https://github.com/KDE/plasma-workspace/blob/master/ dataengines/notifications/ notificationaction.cpp#L54
Would it be possible to expose that signal somewhere?