I was just wondering the same thing for a 2013 Honda Civic - has the screen's (the "head unit"?) firmware been fully dumped and reverse engineered?
How realistic would it be to run fully custom firmware there, and even communicate over usb - for example, if I had a pi running in the car, and it used the usb connection to show things on the built-in display?
It's probably easier to just buy an aftermarket head unit. The only thing you'll need to confirm is that it supports features you care about e.g. steering wheel controls.
Probably more difficult than hijacking the display with the pi. But that's just a vibe.
I'd love to pwn the head unit in my 2014 Honda CR-V just so I can stop it from sending a "Play" command every time a phone associates with the Bluetooth and the audio system was left "on".