Hi Andy, In response to your request for a solar charger one may use while walking; there are several companies selling flexible mat roll-up chargers in all sizes and power specs. I came across them a while back when I was researching on the net and checked again so could send your readers some links- they are even more advanced now so getting one for yourself is just a matter of doing some research on the various styles etc. deciding which you like best and of course paying for it. Though some are not specifically made for hanging from the top of a backpack per-se they almost all have a hook so with some small adjusting you're all set.
Here are some links you can check out. I had found a company before that I thought was best overall but I am not finding it now and the info is on my home pc, however if I find more good info on this I will forward.
Hughes Solar Energy Corporation
A global provider of solar power systems and supplies.
Equipping people, business, homes and research projects with abundant Solar energy through better Products & Know-How.