1. There are a number of dyno to USB converters that all have
different output curves. There are also different dyno hubs that are optimized for different wheel speeds.
2. I did use a solar panel on tour, unfolded across the tops of my panniers. Under optimal circumstances, it could recharge a couple of AA batteries (about 5000 mAh) over a full day of riding. It also stopped working after about 20 days. If my math is correct, dyno hubs will do a lot better than that.