I've tried searching around for this, but unfortunately there doesn't seem to be a good answer.
I'm working on a slideshow in Motion that uses mostly images that fill the frame. However, a few images must be pillarboxed to fill the frame. I had taken to drawing a rectangle behind the image, grouping the image and the rectangle, and then applying a fade-in/fade-out behavior to the group.
However, this doesn't composite them together quite like a nested sequence would in Final Cut.
Does anyone know of a better method? For now I'm simply adding the background (that I would add with a rectangle) in Photoshop before animating.