I haven't tried that setup though I did have a solar panel that charged 4 AAs when I went canoe touring for two months and needed to charge batteries for lights and GPS.

I'd be careful about leaving a charger sitting on a bike without it being fixed somehow. Someone'll pinch it. I thought about solar panels on my touring bike, but fragility convinced me to take a normal charger and plug it in now and then. I was hoping to use a dynamo and switch it on downhills but they didn't have enough oomph.

I met a touring cyclist about 500km west of Townsville last year. He had a solar panel to charge a motorcycle battery. Boy did have some electrical gear with him apart from lights: radio, cassette, and 12v motor and pump for a handheld shower fed from a 5 litre water container. He had a trailer to carry everything and was in it for the long haul - already three years on the road when we met him, so I suppose he needed his small comforts.