Once you enter the DIY space, pretty much anything is possible! I’m sure you could find a solution to integrate the USB power bank functionality (5v) into the “support cell” housing to have a clean unit.
Looks like there are companies that make charger/USB power bank devices that would work with the 18650 cells. Charge the cells and use them in your light when you need them, charge your GPS via USB when you need it!
I could take some photos of my battery setups if you are interested. They aren’t consumer-level polished, but they work and that is most important to me!