I've spend a fair amount of time trying to find a way for an app to modify the main menu (the File Edit ... Help - of the menu bar) of *any* other app.
As far as I know, Apple won't let an app do that. Can someone confirm ?
I don't know about Apple "letting" an app do that. I have no idea how an app would do that in any kind of reliable method. You would have to do all kind of crazy hacks and even then it probably wouldn't work on anything past 10.8. And getting it to work on Catalina, Big Sur, Apple Silicon? Not a chance!
As for App Review, they are very particular about "your app". They require that "your app" conforms to the App Store guidelines. One thing they most certainly will never do is run some other app. They only look at your app and your app's menus.
Furthermore, modifying some other app's menu, even if you could do it, would almost certainly be impossible from the sandbox. Have you actually tried this? In the sandbox? If you can make it work, then don't waste your time on the Mac App Store. Write it up as a security bug and go after the bug bounty. Because if you can make that work, especially from the sandbox, you would really get Apple's attention. They would want to shut that down in a hurry.
Even if you truly believe in the impossible and get it released in the Mac App Store, you won't made one penny. But as a security bug, faking out menus across the operating system, with a sandbox escape, that's got to be worth 5 figures at least.
