That's a non-starter in most countries. Since the car software is tied into a number of important safety features and regulated controls, custom operating systems will never be supported.

There are already massive problems with people miswiring head units to play videos while driving and updating their ECU to spew pollution into the air. You're not going to convince any significant number of people that it's a good idea to allow arbitrary code to run and control most of the other systems too.

Could it be a right-to-repair issue? That seems to be the only legal wrench available for forcing automakers to open up access to anything.

Why should they ?! Do you also want to force them to design their cars so the engine is easily replaceable by a Custom Engine OS so that the community can build their own engines ?!?

That's unacceptable, because intelligence needs a way to steer your car into oncoming traffic if required to do so due to confidential national security reasons.