So what's your solution Kim? I'll need pictures!
Female Tamiya connector on the front luggage rack, spliced into the cable between the hub dynamo and the front light:
Equivalent on the Dawes upwrong:
The charger has a matching plug on its trailing lead. You just plug it in when needed. Only pic I have to hand is of testing this prototype, which doesn't show the connector very well:
Obviously the homebrew regulator can be substituted with a e-Werk or equivalent. (My design works, but the USB-Werk is now cheaper and neater due to economies of scale and having its own custom plastic case.)
The idea behind using Tamiya connectors (rather than something waterproof) is that they're open to the elements in a way that doesn't short the contacts when wet, and they dry off easily. Waterproof connectors tend not to be waterproof when disconnected, and most of the time I'm not using the charger.
Unfortunately, Tamiya connectors do require soldering to make the connection.