For most library uses I think you’d want them to link to additional content that wouldn’t fit on a normal sign. So I don’t like your example of hours and contact info on the front door, but your other examples are pretty good. Unfortunately, until they get really common you kind of need them to provide “bonus” content that doesn’t exclude users who don’t have a reader. I could definitely see putting a QR code in a book so that it links to the author’s blog, or linking to movies or websites for film and web design texts. It would make dealing with clunky URLs much easier . . . as long as you find a reader that works.