Yes, the way I think it will work is it will follow Kirchoff's current law with two currents (battery and generator). The load will draw from the generator first and then battery. A few scenarios on a 1000W ebike:
1. Off the line = load is 1000W, generator makes 300W, and load draws 700W from battery (1/8th of the time???)
2. Getting up to speed = load is 500W, generator makes 300W, and load draws 200W from battery (1/8th of the time)
3. Cruising = load is average 300W, generator makes 300W, and load draws 0W from battery (4/8th of the time)
4. Coasting or braking or stopped = load is 0W, generator makes 300W, and battery is charged 300W (2/8th of the time)
For a high power ebike, the 300W is not that amazing but remember that it is constantly providing a current for charging or reducing load on battery. This adds up over time that you are stopped at the light, coasting, braking or lightly pedaling and not using power. Especially, if you have an efficient ebike motor constantly running the generator helps.