The Wikipedia article on windbelts is not altogether complimentary and suggest that the power provided by these devices is small. However as with any power generation technology, it is possible to use the power to drive an inverter which would put out a constant voltage and frequency. There are even portable petrol powered generators that use this technology to provide a clean sinewave output from a high efficiency but less than ideal generator. I'd guess that would be the way to do it, as you say the wind is variable and getting a constant frequency and voltage out of this (and other) technologies would be very difficult.
