Most of the benefit can be achieved in one step by putting an 'eco mode' button on home chargers that makes it dispense power if and only if there is curtailmen

Most of the benefit can be achieved in one step by putting an 'eco mode' button on home chargers that makes it dispense power if and only if there is curtailment (which can be looked up over network) or the car battery is critically low. Make this an overridable default with some messaging about owners doing their part for a greener future to make it palateable and you don't need the battery to feed back out for the EV to be acting as storage. This benefits the grid operator because they get to massively overcharge for curtailed energy. It benefits the EV manufacturer because they get to greenwash. And the consumer doesn't get a say because they don't actually own their DRM'd remote controlled car and charger.