Interesting. I wasn't aware of this effect. However, my second question would be, is it possible to patch something like this into a Panasonic hub-drive bike without invalidating the warranty? For me, anyway, I'm thinking that separate battery lights are still the way to go. For most other ebike owners though, I'm sure you're right - run them off the main battery.Your main battery will recover enough once you shut down the motor to run the lights for ages.
And for a backup, I've always got my dynohub lights to fall back on - they're not that bad, I just have to keep my speed down.