That sounds like one of those things they display in "World of Tomorrow" style exhibits -- rocket backbacks that enable a man to fly (for ten seconds before running out of fuel), a D battery with a solar cell built right into it that can charge it up (in about a month of continuous full sunlight) ...
Wait -- an idea just struck me. They have flexible solar cells now that can be rolled up. I think it might actually be workable to take the D cells out of your flashlight, unroll the solar cells they're wrapped in (revealing they're actually C cells, but that's okay) -- unroll the solar cells and expose them to the sun all day and run the flashlight all night? That would not only be cool, it might even be practical. It would be even more practical to wrap the solar cell around the outside of the flashlight body, though.
