How about connecting a joule-thief circuit (DC-DC upconverter) to a cap on one side and a small solar cell (like the ones in calculators) on the other? You pull off of the cap to the cell phone directly. If I remember correctly, most cell phones have internal Vregs between the battery and the rest of the circuitry, so that takes care of your OV protection. I'll see if I can build this, and then get back to you.
I then went and built the solarCharge, v.1. This is essentially the Joule Thief with a small solar cell instead of a battery, and a large (50 V, 680 µF) capacitor across the Vout leads. I'm working with the materials that might be readily available to someone who's been able to scrounge through something like an old calculator junk pile, so I don't have a ferrite core, which is probably limiting me significantly. I think what I'll do for v.2 is build without limits to what the parts are, but with a minimum quantity of parts, and track the expense.