Earlier in the thread JML mentioned the Exposure Spark;-
http://www.exposurelights.com/product/000022/sparkI used the same light in CTR, helmet mounted. This is an awesome light for bikepacking, especially for race type events.
It runs CR123 batteries, normally found in cameras. The disposables are best and give 2.5 hrs high (220 lumens), 8hrs at mid (fine except when descending). The low setting is enough to find your sleeping bag / set up camp when you grind to a halt.
I carried 8 pairs of batteries and disposed of them (responsibly) in towns on route. Not sure if these are available in stores along the CT but the weight was not so bad. I was going to take an additional rechargeable Exposure light but decided to save the weight. The Spark alone was near perfect.
This light weighs less than most conventional head torches but gives enough light to ride / descend safely at a decent speed.