Typical dynamo hub output = ~3W. Typical hub motor output = 500W. Typical bike battery capacity = 500wh to 1000wh. A properly designed hubmotor will stop making power above a certain speed to avoid burning out components, so more speed does not get you more than the rated power (3W remember?). So, NO, no one has even experimented with using a dynamo hub to keep an e-bike battery charged because the idea is a complete non-starter.