Some aircraft have powerful strobe lights installed that would, if undamaged, be much more effective.
I think that a handheld camera flash would be a pretty weak signal, but I would certainly use it if I had it. A detachable flash from an SLR type camera would be best, as it will keep going until the battery runs out. By comparison, the flash from a disposable camera would be much weaker, and could only be used up to 24 times (until you ran out of film). It might be worth buying a disposable flash camera and chopping it apart to see if there's a way to get the flash to fire after the film's been used up, though.
I wouldn't bother carrying a powerful strobe for emergency purposes myself; I suspect a PLB would be cheaper and lighter.
If you do have a camera-mounted flash gun, check the manual (if you can find it) and figure out how to get the most powerful flash out of it. If you just mount it on the camera and take a picture, for example, it may read the ambient light and reduce the flash output accordingly. With my Nikon flash, I would set it to manual and set the film speed to the lowest possible setting (although I don't know whether this would make any difference); then I would press the "test" button. (Note that even weak batteries - up to a point 0 will produce the same brilliance of output; it just will take longer for the flash to recycle in between.)
The only way you'd know for sure, I suspect, is to grab a partner and go out after dark and test it, to see how far away the flash can be seen. If anyone has kids in school, this might make an interesting science fair project.
Another thing I would try, although it likely wouldn't do any good, would be to build a mirrored reflector. Think of a cardboard box; remove the top, front, and one side and what are you left with? The back, bottom and the other side, each meeting the other two at a 90 degree angle. If you arrange 3 mirrors in this fashion, then a light shone at them will be reflected back toward the source, regardless of the direction (as long as the light is actually hitting all three mirrors). Yachts often have similar devices mounted as radar reflectors on the top of the mast, I understand; it's the same principle as the reflector on a bicycle. I was told when I was in the military that the French Resistance used these to signal a landing area for Allied pilots coming in at night; the pilot would hold a torch (flashlight) in his teeth and would be able to see its reflection on the ground. (Each Resistance member carried a single mirror, which - in theory - wouldn't raise suspicion if he were stopped and questioned. In practice, I think the Germans would have been very curious <img src="/images/graemlins/confused.gif" alt="" /> ) I suppose it's possible that a pilot flying towards such a contraption at night might be able to see his own lights reflected from the ground. It might be a way to test the visibility of your flash gun; e.g. if you can see the reflection of the flash from 1/4 mile away, then I suppose you can reasonably conclude that the flash is visible from twice that distance (as the light has travelled 1/4 mile in one direction and another 1/4 mile in the other).
I might try other things; for example, string up a mylar Space Blanket and try bouncing the flash off that to see if that's more visible at a distance than the flash by itself.
I think this was a case of somebody making use of whatever was available and getting lucky.
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