The military has looked at this extensively. Their needs for power are much greater than BPers. If I run out of juice in my cellphone, I can't post on FB. If HQ doesn't know a squad's location, they might drop a 500-pound bomb on them.
Focus on long-distance hikers, not on day hikers. You just can't beat an extra lithium battery bought for $1.76 on eBay out of Taiwan. Even for a week or two, I'd just bring 2 or 4 extra batteries.
More bad news: if we go to Ethiopia for some Doctors Without Border gig, I'm packing a solar panel. So the Venn diagram of this application is pretty small - moving far each day, great distances between re-supplies, deep pocket, but no sun, no wind, and no current.