That's completely backwards. Apple does not have rules stopping anyone from doing anything with the Touch Bar. The HIG is a set of guidelines, not rules (which is why it's called Human Interface Guidelines), you can violate the HIG all you want and still get in the App Store. The whole point of my last paragraph was to describe why the HIG isn't what's stopping you from doing something like MenuMeters, the way the system itself works is what's stopping you. The frontmost app gets control of the Touch Bar. If you're not the frontmost app, you can't touch it.