I strongly suspect that a small solar panel would not provide enough power to charge the headlight's battery to any significant extent. Stuff like that may be available in a year or two but not yet. You'll need to plug this battery in, for sure. You may want to buy a spare battery and carry it for occasions when this one discharges on the road.

It's a lot of work, to my thinking. I'm super pleased with my dynamo powered systems. I only rely on battery powered headlights briefly and infrequently nowadays.