Ahhhhh! Okay, this is really bizarre.
If you have your camera and your instance, and you set the original as the render camera and the instance as the projection camera, it works in Preview but not in frame rendering.
If you set the original as the projection camera, and the instance as the render camera, it works in frame rendering but not in Preview.
I can't figure out how to get it working in both Preview and frame rendering. Weird.