The statistics I've seen (on California[1]) suggest that homeless folks are generally locals, not migrants? But I agree that just providing some usb ports on public transit (likely also in stations or at stops) would be a low-cost/high-benefit approach. For human-powered approaches, I will say that old OLPC hand crank was a good design, since it clamped to a table or railing, which allowed for clever 'hacks' like hoisting a bucket or water bottle and letting the weight coming down turn the shaft [2]. Still less practical than just plugging in, to be fair.

1: https://californiahealthline.org/news/article/california-hom...

2: http://www.olpcnews.com/hardware/power_supply/two_awesome_ex...