Fire Ray

This Blender addon will insert an Empty in a specific place on a tracked camera.

Once installed, the addon wil appear in the viewport's tool tab.

USAGE: You need to have a tracked camera with a background image. Place an empty where you need it. Press the Fire button. This will create a line that goes from the camera to infinity, passing through your empty. Move to another frame. Using snap to edge, move the locator so that it matches the area where you placed it before.

It's much easier to understand it you just look at the clip here: https://www.youtube.com/watch?v=RmL6dq4NcQo&feature=youtu.be