There was a similar thread not 2 days ago. only way to do this, is to ensure that everything on screen, including 2d is in the viewport, and that the designs would fake a smaller viewport. This way its possible to make it look like a postcard that you can turn, that you can distort the “fake” viewport as any other 3d element.
I know its hard to forget some of the display list concepts, but keep in mind that the Stage3D is under the display list and that it’s rectangular region that cannot be something else than a rectangle. The art in your case would be to give another impression.