Thanks for you replies, and the useful information.
This light also comes with the 3xAAA battery holder, so perhaps NiMH AAA in the holder would be a safe way to experiment with solar charging the light directly.
In the Rechargeable Battery forum I asked what would be the best way to solar charge an 18650 battery, and I received a few replies so far.
“Premium Solar Charger setup: Big #*s solar panel 50W+, Morningstar Sunsaver lead acid battery charger, 12V sealed lead acid battery, Li-ion battery charger with 12V input. Charge 18650 batteries morning, noon, or night.
Budget Solar Setup: Not so big solar panel of 5-10W (foldable?) and with USB output connection. USB single 18650 charger like the Miller series. Charge while the sun is up.
Nerd Solar Setup: Solar Mini-panel 1W, and diode connect to 18650 battery pack, and keep track of voltage with a microcontroller circuit. Wait three days or more for a not quite full charge (CV only) A Solar Rechargeable FlashLite - Done! (2014 BLF Handmade Contest)