The higher the voltage, the more interesting it becomes I think, but the higher the voltage the more insulation you need IIRC. Perhaps you could only insulate the bit at the beginning and end, letting the air around the cable be the insulator for the rest, and only send electricity through the cable while it is fully reeled out. You’d have some load at altitude to absorb electricity for when the cable is not fully reeled out.
I think this could be applicable to any system that has a fixed point at altitude, so excluding the Makani design.