As I know, all Auto market (not only EV) is extremely regulated (mostly, because of safety measures), so it is not much opened.

I think, it is unfortunately, but not exists official market of custom ROMs.

For about custom ROMs, situation very much like custom ROMs for Canon camera - they don't implemented totally new features, but with hacker methods, unlocked some hidden features, already existing in hardware and software; also exist similarities with Smartphones custom ROMs - for many Smartphones only possible to reset firmware to something like "very new device without traces of wearing", plus some copies of features from same model but other region, or mods (unlock features).

Significant difference of EVs, for them much more frequently used things like "power or torque limited by software", because to faster get certification, producer could limit torque to be equal to ICE original vehicle (usually, electric motor have much higher torque than ICE with comparable other parameters), and in such case don't need to much bother with certification, as in many countries such limited electrification come with very simplified procedure.

Sure, could exist lot of hacks for entertainment system.

And many hacks you could DIY with things like USB to CAN bus controller.

What also interest, exists large market of totaled EVs in 3rd world countries, where in garages literally could refurbish Tesla from trash (suffered extreme collision after which owner decided to utilize machine, but some guys bought it as trash and resell to other country, and it got second life, and I hear many cases, when previous Tesla owners once began get messages from their account but from whole other country).

But that's nearly all, except entertainment system, other parts of EV usually considered highly regulated, so you cannot account for much customization of them.

And level of hacking is very much depend on brand - as I hear, for example, Daimler including in machines "CAN bridges" between entertainment system and ECU, and classic CAN connector could only see entertainment system messages, but access to ECU severe limited (cheaper brands usually don't use CAN bridges).

And sure as with other embedded tech, with time more new designs use locked boot and signed blobs, so with 10 years old machine you will definitely get more access than with brand new.

BTW Tesla is by definition special case, as their electronics could be from different manufacturer in each production batch, as this claimed as feature, because of which Tesla less suffered from covid related issues (when many electronics manufacturers severe drop supply). Sure, this mean, for other manufacturer could be other hack (and early batches hacked much deeper than more modern).

Ask HN: Do custom ROMs exist for electric cars, for example Teslas?