I think there are possible solutions here for a library, off the top of my head, taking a picture of your face when dropping off the codes, so that when you come back and ask for your codes, the librarian can ID you against the picture they have. Basically what is done when verifying your ID card/passport when you travel/go to the bank etc...

It wouldn't be a librarian doing someone a favour, but rather a service that libraries provide.

This could be a great evolution for libraries. They are already a distributed, public system, that people in general trust, but their role in society has changed with the rise of the internet and online services, and this could be a really useful role they could fill.