The beam was/is about like a dinky C-cell hotwire flashlight. Little yellowish disc, barely visible reflecting off asphalt. They were always more to be seen than to see anything. That’s why I had/have mine aimed almost straight forward, only a slight dip.
They’re more like constant-wattage devices, load-limited by the winding resistance. Without a load, voltage shoots up sky-high (k·ω, proportional to rotational speed). With increasing load, voltage decreases to just supply the required wattage the device is drawing for the given speed.
So absolutely, you could charge even series Li cells as the voltage would rise pretty fast at even moderate speeds, but at a lower current. You’d just need to pre-regulate the voltage to make sure you don’t fry the charging circuit. And if it’s not spinning fast enough, it’ll just slow down the charging.
The nice thing about hotwire bulbs is that even at a small fraction of their rated voltage, they’ll still throw decent light. A 6V bulb will start throwing light at 2V or less, albeit not nearly as bright. So just start pedaling at all and it’ll start glowing.