If it was "burning" batteries in the beacon that started the fire, and they were still involved when the fire crew arrived, then it's unlikely that anything short of cooling the batteries below their reaction temperature would stop the energy release. I understand that the failed batteries do not require external oxygen to start, or continue, to release heat. Presumably halon will quickly stop the burning of the aircraft structure in air, although unless it's also cooled, than the fuselage would presumably reignite if it stays hot and the halon drifts away.