Responding to this ancient thread in case anyone else finds it via Google search, as I did, and can use this simple solution I used. I think this is similar or identical to what Ian described, but I couldn’t quite understand his description so maybe this will help someone. This is in CC 2017:
1. Create a null object and keyframe its position to the first and last frame of the uneven pan, so its anchor point is attached to a point on the image (e.g., I attached it to the top of a particular tree). By default the null object will move evenly across the frame, so it won’t always be in sync with the point you picked.
2. Use Motion Stabilizer to stabilize the footage to that point (in my case, the top of the tree).
3. Set the footage layer to have the Null layer as its parent.
4. Scale the footage as needed to hide blank frames at either end of the pan.
Voila!