I'd like to have a background image in the NSSavePanel which I need to satisfy macOS Sandbox file export requirements. Such background image would be reflective of the overall imagery I use in the other application window and sheets. Apple's default NSSavePanel image style is okay for an app that cares little about its style, but my app is directed to video editors who can appreciate an artistic rather than mechanical atmosphere.
I've been able to use the panel's accessoryView property but it only appears as a differentiated section in the panel separate from the panel's overall style.
A simple background image for the panel overall capability would completely suffice, but it doesn't seem possible according to all the Apple Developer documentation. Maybe there's a practical HACK?