I use a waterproof, marine-style, cigar lighter socket too. Before I fitted it, I used to use emergency mobile phone chargers as seen in all the £1 shops. They work surprising well though you'll need a supply of batteries.
Could you not use a bicycle dynamo lighting circuit to power a socket, maybe just to supply a feed to a battery charger in case of damage to your phone. My dad used to use a dynamo on a bicycle years ago but I'm not sure if you can still buy them.
Saw one of the Freeloader chargers when we were last on a trip. The lad just put it in the map pocket of his tankbag. Seemed a really good idea until I read the reviews of them when I got home.