Ok gang, I need your input.
TL;DR: Cheap, clever, and easy? Or proper design that cost more?
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When I originally costed out the design, I hadn't carefully looked at the regulation voltage needed by LM317 in constant-current mode. It turns out that you need about 3V of overhead, and the power dissipation is pretty significant.
So I went back to the drawing board to look at alternatives.
Doing It The Right Way, of course, is to have a switching regulator in CC mode. It is theoretically the most satisfying, because you know you're squeezing out the most out of the batteries. But the BOM cost goes up a bit, and there are much more parts. Too many parts to get first-time soldering-iron users to get right in a limited amount of time. We could build the circuit up ahead of time, and leave the soldering to only hooking up wires.
Then, there's the Clever But Sloppy way using a PTC resistor, which turns out to be a really cheap way to do it. It's just the LED, the PTC resistor, and the battery pack. Cost goes down, too. But there's significant power dissipation in the PTC resistor, the current regulation is very sloppy, and there's far less uniformity of brightness between units, and over battery depletion time. But the bonus is: it's easy! And cheap!
So the practical side in me says go with the PTC.
The engineer / scientist side in me says go with the switching regulator.
And I think the simple moderate-cost LM317 approach is my least favorite -- if only because you'd otherwise have to lug around a 5-battery pack to make it work right. :(
Oh, and there's the traditional "LED + resistor" approach -- but that's not practical for high-bright application like this, since the system is highly sensitive to LED Vf and resistor values.
Joseph