Mr. Fly

While that is technically true, given the same argument, one can say the same of most (if not all) USB charged lights. Basically, what you're doing is running a AC>DC convertor off a dynamo hub which will then charge the light through its USB charging port. This may not be the most practical because I don't know how well that connector will take to that kind of "running" stress. It certainly doesn't look too sealed at that point either.

But that begs the question (and I apologize for the topic drift). Discounting 5-minute standlights, why don't manufacturers make a dynamo-powered light with significant power reserves (30 minutes at full power?) but can also be charged by an alternative source (e.g., USB)? Dynamo-powered lights aren't exactly big power hogs so having enough reserves to run for 30 minutes shouldn't need too big of a battery (a single 18650?). This kind of light will be useful for tube changes and such.