I need to limit quantity of some items to only one per customer (because the info they enter must be unique). They can select the item again and then enter its unique info; that works fine. I just want to make the Qty box 'greyed out' on only some items.
You may want to consider creating an "Add-On" for the product. This requires the customer to enter data for one-item only with a custom feature (like monogramming or engraving). See if this will work for your purpose.