Is there a way to make Scene types with per-instance roots?

Just been watching some WWDC 2020 videos again, specifically "App essentials in SwiftUI". I wondered about the two different scene types shown. Most of the time was WindowGroup, but the end mentioned DocumentGroup.

The DocumentGroup scene type allows instances to have separate root objects. But the examples of WindowGroup used a shared root, even though each scene kept independent values of where they were in the interface. Is there a way for a Scene type to be configured to allow independent roots? Can that be added if not? (Like DocumentGroup, a scene type allowing independent roots would need a separate View type to represent the "open-file dialog" equivalent to choose a root.)

This is a rewording of a previous post. I've also added an official feedback for this: FB8307528.