This may be stupid, but:

I wonder if any electric car engineers thought of building a dynamo ratched into the handbrake lever? Keep the brake electric, but in the case of flat battery then act of pulling/pushing the lever would generate enough power to trigger whichever relay controls the park brake.

While I can see the benefit of an electric park brake (no time wear and tightening of the cable for example), I'd absolutely hate to think what would happen if you have a relay fault in the park brake causing it to go off while driving
After all, a relay switch is still susceptible to becoming faulty but at least a mechanical handbrake's default position is "off" rather than either way in the case of a relay.