I don't think that this can work well, although it might for window-relative cursor positions, it probably won't as soon as absolute cursor positions are involved as applications will likely not expect to handle cursor outside of the virtual screen. Moreover, some events such as touch input use a 0-65535 range for their position, corresponding to the virtual screen dimensions. For those, it's simply not possible to have coordinates outside of the virtual screen.
Perhaps a better solution would be to have an option in win32u that can be enabled to constrain windows that are partially outside of the virtual screen, and move them automatically if they can be. Windows that are fully outside of the virtual screen should simply not be displayed.