This little script you can attach to your corner pin points to nulls in 3D space.
For example, lets say you wanted the “Upper Left” corner pin point to be bound to a 3D null called "tracker 20. You would alt-click the stopwatch on the specific corner pin and then type this:
L = thisComp.layer("Tracker20");
L.toComp([0,0,0]);
That would force the 2-dimensional corner pin point to align to the 3 dimension position of the null. Rinse and repeat for the other 3 points.
IMPORTANT: You need to have your graphic /video to be corned pinned precomped inside a precomp that is the full size of your main comp and scaled to fit that precomp that it's inside of.
If the nulls go off camera you might have issues.