[fmntf]

That's unrealistic. A car contains several ECUs. Each ECU has one or more MCU/CPU. Every core runs its software, 99% of the time documentation is available only under NDA. Reimplementing the software for just one MCU/CPU is challenging even for who is part of the industry.

[smolder]

There have been 3rd party ECUs for decades, "piggyback" fuel and spark controllers which take over functionality of the main ecu, devices to disable rev/speed limiters, custom transmission controllers, and so on. I wouldn't discount the aftermarket's ability to develop something that fulfills owners' needs. (Post-warranty)

[detaro]

If you wanted to go for "replace everything", sure. But that's not what you need for unlocking features and maybe some UI stuff. (I still suspect it won't be that common, but I'd assume it to be much more in line with other jailbreaking projects)