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.
I have indeed done that; that is how I capture the info we need from the buyer. Some items have 4 or 5 add-ons.
The problem is that they can still revise the quantity...
Doesn't happen often enough to really sweat over, though. Just hoped we could make it work perfectly; foolproof.