Not without voiding the warranty. Incidentally, the exact same thing is true of the iPhone.

Do you think it's right to expect someone to warrant something with work after you have modified it beyond how it has been designed to be modified?

Is Ford violating anti-trust law by not letting you change the software (GSP app, for example) that runs on their dash system?

What does Ford do differently with respect to software on their, say, entertainment system or ECU computers that Apple doesn't do with their iOS devices to allow you to draw this distinction?

You can easily replace the ECU of the car.

Meaning that you can easily replace an entire computer system with another entirely different computer system? What stops you from doing the same with your iPhone? (i.e. buying, say, an Android device)

Have you personally ever done so? I have, and it ain't so easy, especially if you are not just swapping in a piggyback system, but actually installing a whole new ECU.