Usually, in other 3D programs, such requirement is solved very simply. Just make a camera to be parent of your widget. Move widget locally (usually in Z direction) in Unreal front would be X direction, so your widget is always in front of camera. Just adjust its distance from camera and its scale. That distance determines how large space will be for other object which can be in between your object and camera to occlude your widget. If widget can be put in camera space, that would be the same. Local X, is the offset from camera. So your widget “travel” with the camera in the scene, it is really in 3D scene space, but viewing from that cam, it looks like it is stick constant on the screen.
I guess, same can be applied in UE too