It's possible to build a dynamo that provides a lot of power, but it would suck too much kinetic energy from the rider.

That chain-based dynamo seems close enough to what you're after, especially if it is energy-efficient. But bear in mind that there is a practical limit on how much power to expect from any such system because a rider is probably not willing to pay a 20-watt kinetic energy price for charging. And given that charging is available nearly everywhere from a power outlet, you're solving a problem that not many people have. Most people are near an outlet every day. Even if you're away for two days, you can get by on two little external batteries. I carried two on one long ride I took. I ended up depleting the first but not using the second. I kept my phone's display on for more than six hours. By the end of the ride, my phone's charge was at 100%.