It's a strange one to explain. I think that you almost had what I meant right first time, except all the video's wouldn't play from the start of the composition to the end.

The video layers only start and end as and when they will come in and out of the final standard size output frame.

Initially there is a bit more work. It's just really creating one more comp and working within that before putting it back to a standard size comp.

I'm sure it would cut down on motion tracking and tuning the motion afterwards.

Both would work though, it's just another means to an end.

David.