The daytime running lights on the bikes get noticeably dimmer when the 18650 lithium cell voltage drops below 3.6 V, so I picked up a quartet of Tenergy protected cells and ran ’em through the battery tester:
As with the ATX cells, the voltage decreases almost linearly with charge until it falls off the cliff near the end, but these have a higher terminal voltage throughout most of the curve, which is a Good Thing for LED flashlights.
These four seem to have about the same overall capacity as the ATX cells, so we’ll run ’em all in sequence and see how long they last.
The Smell of Molten Projects in the Morning 
Is it practical to use something like a joule-thief so you don’t get the dimming?
Now that I know what to look for, I could DIY a flashlight with a built-in boost converter to get more-or-less constant light out of the declining voltage. It’s probably not worth disappearing down that rabbit hole, though, because the lights seem plenty bright even at half-power.