Ok I honestly think that this is a great idea what if you took a fender and filled it with batteries. From there you could run a cable down to the charging point on the Onewheel. Now the only problem I see with this is that in order to recharge the fender you would need a separate cable I don't think that you would be able to split off of the main charger for the one wheel. That being said if you wanted to you could also put solar panels on top of the fender so that when you are riding on a sunny day you would be able to gain some power. However in order to keep the cost down it would be best to go without them, but hey its an idea either way.
I am gonna try and contact FM directly about this and personally try and see if this is even possible but if this is something people would want them them know.