Here is a tutorial I did on how to do an offset track in Motion. I guess it depends on the footage, but a one point offset track is easy to do. Just take a look at the tutorial. My concern is the rotation or skewing caused by a 4 point track not lining up with anything other then the picture frame because it is being hand held. Meaning, if the hands move the picture frame the background will be at different angles compared to the picture frame. It could be a lot of fun to take a look and see if it works. Take a look at the tutorial I linked to at the start of this paragraph.
Stephen Smith
Utah Video Productions
Check out my Motion Training DVD
Check out my Motion Tutorials