This is not exactly what you're looking for, but I have an external battery charger and 2 spare batteries that I keep charged up - so I can be going around, and if the battery goes close to dead I simply replace it.
I know you want a charger, but thus far this is the closest I can suggest.
Also, a regular dynamo won't do much, but if you somehow rework it with gearing so that it is spinning much faster than actual velocity, you should be able to get sufficient amperage out of it to charge the phone - and then using that device that you used earlier would also be used.
In the end, it might just be easier to go with the multiple battery option.