Yes your distinction is a good — and important one. I have been working on several projects where I am creating “motion” paths for objects in 3D space. What I have resorted to doing is using a null–because I can more easily see it. Then I pick whip the z position over to my primary target–or emitter as the case often is.
I long to be able to change the camera angle and work from the top with a pen tool and create my vertices across the Z axis. I think the short answer is: I can’t. Period.
It’s not a specific project-oriented question, but one of general work flow.