IAANAL, but I don't think even that is true. I think it actually is legal to allow it to take control of your car, provided you are still sitting in the driver's seat and can take control back at any time.
Can you GUARANTEE that you would be able to do this in a modified car with such a setup? You might be able to do so if we are talking about pure mechanical overrides. With blackbox software that's imho next to impossible. There will be no public documentation on what the cars systems will do if the receive some commands on the bus from a non-approved device and others from the driver. Even if you can observe through testing that driver inputs win the system might still behave different in some edge cases.
but if you're going down the highway and the AI decides to suddenly swing the wheel far right as quickly as possible, no human being would have reaction time good enough to recover without crashing. I'd be pretty concerned if you could add an autopilot to your car and drive it around on public roads without any sort of demonstration of safety.
"The EPS controller in the car limits the torque to a very small amount, so regardless of the message, the controller cannot jerk the wheel" [1]
[1] https://github.com/commaai/openpilot/blob/master/SAFETY.md
I can put a Chevy engine in a Ford, and plenty of people do much more than that, e.g. homebrew EVs, etc.