I know with the Leaf that would be relatively easy, because it doesn't ensure current from the battery matches the current going to the motor, so if you add a p

I know with the Leaf that would be relatively easy, because it doesn't ensure current from the battery matches the current going to the motor, so if you add a plug into the HV and inject power so the battery current nets zero, it can drive indefinitely. The gen2 (non-plug-in) Prius throws errors if it detects current out of balance so it's a lot harder to hack. So whether any given car could easily be hacked to use a generator is up to the particular implementation and us hackers don't have those Rivian vans! Though, I wonder if anyone has hacked the pickup yet?