I posted this on the Adobe forums a couple of weeks ago: http://forums.adobe.com/message/3021469#3021469
Basically I would like to mimic Nukes' "PointTo3D" node. What it does is that it can find the proper z-position of an object in your 2d plate. You basically select the 2d position of your object in three different times in the timeline. Then you plug in your match-moved camera (you must have a camera) and press go. It somehow calculates the position with the three points and the camera. Very handy when you don't have a point cloud to your camera, or if you have to find a position for an object where you don't have a null.
Would this be possible to calculate via expressions somehow?