Charge CR927 battery with small solar cell?
lithium coin cells can take a bit more abuse than their larger, more flammable cousins. with the limited current that a solar cell that size would generate, you could probably get away with a direct connection to the battery with a pv cell with a high enough voltage (and series shottky diode) and just put a small zener diode in parallel with the battery to limit the peak voltage. the powers we're talking about are so low that burning the extra milliwatts as heat shouldn't be an issue. Or if you want to get fancy, both TI and Linear have energy harvesting chips which will pull the maximum possible power from the solar cell and produce a regulated voltage to charge a battery/supercapacitor.