3D to 2D ScreenSpace is a free After Effects utility from Julian Herrera for translating a 3D position into a 2D null placed in its relative 2D screen-space. You select a 3D layer, then choose how the connection is made between the 3D point and the new null.
Pressing 'Link' ties a new null to the 3D point with an expression. Pressing 'Bake' converts that expression to keyframes straight away, giving you a keyframed 2D null instead of an expression-driven one. Both options work from the same starting selection, so the difference comes down to whether you want a live expression link or baked keyframes.
The tool runs on legacy versions of After Effects, including CS6, CS5.5, CS5, CS4 and CS3. A demo video is available for those who want to see it before installing. It is listed here as free, with the initial release dated September 6, 2016.