Using hub-dynamo for both lights and charging, with priority on lighting
I suspect you will find that most of the chargers that run off hub dynamos take this into account. For example, http://www.thinkbiologic.com/products/reecharge-power-pack in the (pdf) manual say:


  The ReeCharge is charging when the green light on the side of the
  ReeCharge is lit. NOTE: The green light is only on when the wheel
  rotates fast enough to provide the ReeCharge with a current for
  charging.


In your position I would buy one of the charging setups and see how it works in practice. If there's a problem, you will need to add a voltage sensing switch. Either way, you need to buy the charger so there's no harm buying it.

A voltage sensing switch is easy enough to make, either using a zener diode or just a voltage divider into the gate of a transistor. http://www.instructables.com/id/Voltage-Controlled-Switch-1/ is one example (but without a bigger transistor on the output that will not handle the current your charger uses). I would go with the simpler approach of buying a power transistor that can handle the 500mA your dynamo puts out, the 15V or so maximum it's likely to put out. Use a variable resistor to work out what voltage divider ratio I need to make it work, then substitute a pair of resistors in the final version. That's 3 components and it should be easy enough to put them in heat-shrink or epoxy to make them weatherproof. You'll probably pay as much for the connectors as the components :)