Custom menu

[Iniquitatis]

Let's assume I have a TC, and I want to reinvent the wheel by writing custom menu code from scratch* without using MENUDEF**.
At first everything is going fine, because GZDoom gives you an access to the 2D drawing routines, mouse/keyboard/joystick input, etc.
But then I need a "Quit" option in the main menu, for example. First thing that comes to mind is to call some native function, which will terminate the engine. ZScript doesn't have this function, so I can still use CCMDs. And only ZScript class in the entire engine that has access to CCMDs is an OptionMenuItemCommand with its private DoCommand function.
The only thing left is to call a Menu.SetMenu("QuitMenu"), but this menu*** can't even be redesigned properly.

So, which options do I have, besides modifying the engine source code or any of the native actors?

* — It just doesn't share many similarities to the original Doom menu.
** — Actually, with MENUDEF, but just for reassigning class of the "MainMenu".
*** — Same goes for any of the generated menus.