So you would play Weakness Policy and Bursting Balloon in the same deck? It’s not a terrible idea but I’ve found that the only deck that can reliably attack Trevenants turn after turn under Item Lock is Dark type decks, or Manectric, and Manectric is just a completely lost cause. Rayquaza might be the other deck that can and it’s too vulnerable too Hammers.
If you see merit in it, try it out and let me know how it goes. But I don’t think Bursting Balloon really adds much to Trevenant if you’re expecting to play against as much Dark as I am.