Adjustments are not really any different from any other 'things'. They've got a checkbox, have the ability to have a couple dropdowns or text boxes, and can run any number of scripts, and bootstrap on any various other things onto the hero, or other containers. For this, you'll be adding a tag to an item. The best way to learn is to copy existing adjustments. In this case, the adjustment that would likely be the best to copy would be the Equipment Broken adjustment. It is already set up to select from all equipment on the hero, and the first Eval Script in it adds some working to the name, and adds a tag to the item, pretty much exactly what you want to do. You can throw out the other scripts from the broken script, as they are for setting reduced value and such to the broken item.
Then, when you actually use the adjustment, you would add them on the Personal tab, under Permanent Adjustments. select the item, and enable the adjustment. That should add the tag, and then when you equip said item, it should not show as a validation error.