One of our libraries came up with the idea that it would be cool to allow library users (and presumably staff) to put things on lists that are not in the catalog.

One possible way to do it:

in the lists interface, have a "add your own item" form, that allows entering title, author fields.

In the list display, list those user created items with the title and author linked to searches for that exact title/author string. The idea being that a user could put a book they knew was coming on their reading list, but that wasn't already in the catalog, and easily search for it later.

Another example would be for librarians to use this to create award winners lists of items that were award winners, but displaying title/authors that the library didn't own alongside titles/authors that the library *did* own. 

I think there are more applications for this - I recall there were several more when I floated the idea originally in the IRC channel.

It's a nice-to-have, not a must-have. Pretty cool feature though, I think.