Toph has trained some of her force to metalbend. They're obviously not as talented as she is, but they exist. Question is, would Azula be able to get from Toph that sort of detail? I don't think it's necessarily unlikely, as while Toph is presently loyal, that doesn't mean she always will be.<br/><br/>The device itself isn't all that far-fetched. As there are particular points that can always be hit to disable bending, it stands to reason that keeping those spots permanently hit would permanently prevent bending. Would Azula consider that Toph or some subordinate metalbender might free someone wearing them? If not, then there's no reason to purify the metal.<br/><br/>For context, in the fanfic, there were heated discussions of what to do with Azula. Most wanted her executed to make a point, but Ty Lee, under the duress of the situation, decided that "castrating" her was better than killing her and proposed the manacles.