Is there a way to make a checklist of all the wardrobe items you have? Is this even the right place to ask?

You can make one with an account on Subetalodge :D The collections page already has Wardrobe as an option; if you go there (from the first dropdown at the top of the page), then to the Mass Add page, it will give you a link with instructions how to add items from your wardrobe c:

Thanks. Will it still work if they're not in my wardrobe?

Not from the wardrobe xml link, but you can input the source code from any page to add them c: