hikermor, yeah, I know about the niteize strap. I asked about it further up in the thread, but chaosmagnet says it's not so great. So I was trying to think of something with more control. Maybe a simpler answer involves some velcro stuck to or wrapped around the light, so you can control the up/down angle. Also, the niteize strap is much too heavy when we are trying to keep the weight of the whole system under 1 ounce. The Niteize weighs 30g all by itself! (New url: https://niteize.com/headband-flashlight-holder ).

Re cost: there's a difference between an expensive light for frequent use, and an expensive light whose intended purpose is to sit unused in a kit for years and (like a fire extinguisher) hopefully never be used. In the second case you have to consider whether that is good resource allocation. Also, it may be better for this light to be mechanically switched to eliminate parasitic drain, so you can leave a battery in the light. I don't know how the HL10 is set up in that regard. This issue is not crucial since you can store the battery outside the light, but that's more fiddling before being able to use it, and more space and parts in the kit, so I'd rather avoid.

Added: for caving, a light and a backup are not emergency supplies, they're necessities, especially the primary! So you would expect to check both lights before going caving, i.e. you can use rechargeables for both and make sure they are charged. Plus the weight is less important. So that gets rid of several constraints of this thread (10 year shelf life, super lightweight, super compact). Plus you'd usually cave with another person, so that's two backup lights between you. Thus I think caving doesn't count in this search. I'd be interested in hearing caving stories involving multiple light failures, and other survival stories where lights were required. I'm a flashaholic obsessed with lights for their own sake, but I think we are overestimating their importance in practice. People did AT through-hikes with just a Photon II. And the incandescent lights of not that long ago tremendously underperformed any modern LED stuff, yet they sufficed.

Added 2: Sadly, the LRI/Photonlight accessory hat clip (the one with the swivel ball) seems to have been discontinued. It always looked a little fiddly (maybe they had breakage problems) but it would let you use the Photon as a handsfree light by clipping it to a hat, pack strap, or whatever. I never tried it but it might have made the Photon into a usable superlight headlamp given a thin elastic headband. Oh well.