Actually, if you wanted to get all gadgety, a solar collector with a feed would heat water that could be run through the hives, and radiate heat inside. One large collector, tubing running inside, running to pipe in the bottom boards...
I can see your gears starting to spin there BlueBee. A shallow box with insulation, plate glass on top "wax melter" style, painted black on the inside to absorb heat, and maybe a PV panel to run the pump. Real problem here is regulating the amount of heat.