
a 1 inch wide strip of leather ( an old belt ) and a pair of vice grips .
Wrap the leather around the seized bit and have the 2 ends together , then grip both ends of the leather with the vice grips ( in a straight line wiht the ends of the leather belt ) and slowley try to undo , this is like an oil filter remover on a car engine and it will apply lots of torque without any damage .

worth a try . with the heat and cold options already mentioned it might get it off .
Brian.