I think it would be really helpful if the Upload zone had a shortcode much like the custom image does. The fact that you have to click on the upload zone in the canvas is often not an option as far as usability is concerned. Especially on mobile. Alternatively/additionally, being able to separate designs and custom images to have different replace values would also hugely improve flexibility with the designer.
This would be great. I came into the forum trying to find a way to do this.