I want to be able to start an Activity that is not part of my app while the device is password locked. How could I do this, if it's even possible?
Note: I am well aware of putting getWindow().addFlags(WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED); within the onCreate of my activity. This will not work, though, as I'm not starting my own activity, but a 3rd party one that is outside of my app.
It will not work if you are trying to launch any third party app over lock screen. As you have observed yourself, you need to set the window flag to ensure that activity get launched over lock screen, there is no way to ensure the activity from other third party app are also setting the same flag.
In case you are building a feature bundle where each feature is nothing but a different application then you will have to ensure that all the entry points of those feature do set this window flag. Best thing would be to declare a BaseActivity which sets proper flag on creation and let all the feature dev team use this as base class for entry points.