This has turned out to be a very interesting discussion. I can't see why someone can't build a "Hybrid" dynamo set-up that would incorporate a small Li-ion battery back-up ( for those long slow climbs when you are just poking along ) that would automatically adjust the output of the lamp by bleeding off just enough current from the aux. battery to prevent the dreaded flicker phenomenon. Done right the circuitry could be designed to kick in a minimal current when the dynamo is just not outputting enough current to meet the needed minimum LED current requirement to help eliminate any low output flicker. Even better if this was programmable by the user so he could either set the level at which output the battery would kick in. Now add a feature like momentary remote high beam and now you have something even more useful.
Super capacitors are great but they to need to be charged as well and I doubt they could hold a charge as long as a Li-ion cell. Yes at some point you would have to charge the Li-ions but with the minimal current drain they would likely see they would likely only need to be charged ( externally or though a USB cable ) only a couple times a year ( depending of course on how many long slow hills at night you ride on a regular basis and how bright you set the current by-pass for.. ). Judging from the size of some of the dynamo lamps I've seen online they could likely make one with enough extra room to house a couple parallel configured 14500 Li-ion cells ( 1500mAh total ) or a similar single cell Li-po cell ( 1700mAh ). The Raveman CR-500 lamp I own uses such a battery. On it's lowest steady output mode ( Eco-Mode ) it claims to provide 16.5 hrs of run time @ 50 lumen. When I turn the CR-500 on it's eco-mode I find it provides more than enough light to see if I'm only poking up a long hill at 3-5mph.