"Trivial to do" likely isn't trivial for you or me, or even "trivial" at all. A lot of information, technical skill and access to electronic guts would likely be needed. JLR could do it, of course, but only after dedicating effort by an engineer or small team, and they would have to believe it's worth it.

I do think building in these features will be of increasing interest to car shoppers, especially a "sentry mode" that watches while the car is parked.

But "dash cam" systems are more sensitive, since they record the driver's own mistakes and violations as much as those of others. It's unclear if you really want the car manufacturer (or the police) to have easy access to that. We have seen how often Tesla grabs the driver's own dashcam videos to defend itself - and to blame the drivers!

I'd suggest going with stand-alone commercial dash-cams. So much the better if they come with encrypted recording so it's your choice who gets to look at it.