You could edit the tables in Reference.py.
DefaultObjectData['Chaosword']= [OBJ_WEAPON, 5, 0, 2.0, THR_STRAIGHT, [W_FLAG_2W,PI,2000,750,100,100
If you change the brk data in the last number (in this case 100 I have highlighted in purple) to 0 you may find the sword will become unbreakable.
I say 'may find' because I'm half-guessing.
You'll have to do the same to all other weapons you want unbreakable, but I think it's easier than trying to hook into functions.