I have to question using an 18650 battery in a headlamp. Serious backpackers would have to carry extra C123s. Unless there is a solar charger for 18650s. Most popular headlamps, Fenix's line for example, use AA batteries in single, double and triple battery combinations. AA batteries are easier to find when in parks or off the beaten trail, and there are a number of AA chargers that run off solar panels. The Goal Zero Guide 10 Plus Adventure Kit Solar Charger and the Powertraveller Powermonkey Extreme Solar Battery Charger are liked by many hunters, back packers and RV types.
I have found where anyone has tried charging 18650s off of a solar panel.