I take it it's not possible then? Do I need to put a feature request?
This is quite an essential feature for full Wayland support!
On wayland, window decorations are part of the client window, i.e. the application itself. I'm not sure your opinion that removing part of the window behind the application's back is an essential feature will find much support, but you can file a feature request if you must. Note that if you do, you should file against GTK+, not mutter/gnome-shell - on wayland, it's the toolkit that adds default decorations to windows that don't specify their own.
