> From my understanding such a program is fundamentally against wayland design philosophy
Against the philosophy but, I think, not completely impossible. Wayland is “just a protocol”, so somebody could write a Wayland server that runs in a Wayland window. Then, you could draw anywhere in the outer window, including outside of the inner one(s).
Make the outer window not have a title bar, etc. and run in full screen, and you’re done.
Isn’t that what XWayland does, except for the fact that it runs X Windows windows inside a full screen Wayland window?