A gas fired kiln could be one solution as you have pointed out. If standard fire bricks are used in place of IFB the kiln will hold heat better and there may not be a need for a PID controller. As the kiln will very slowly decrease temperature that will match annealing needs. But it will take some research to find out how well this works and a proper design for the kiln.
If auto or truck batteries are in the mix, then it could be possible to use a bank of say 10 in series to run only the kiln elements. Some calculations will have to be made to determine if the amp hours of the batteries match your needs. Then the output circuit of the controller will need to be modified to handle DC in place of AC.
If the kiln is in the range of 1000 to 1500 watts then the purchase of a small generator for kiln use would be the simplest solution. In terms of generator power this is not very much. And if propane is less costly than gasoline then consider a propane powered generator.
