It would be doable, and libraries have been amenable to this sort of thing, like not handing over records about checking out books. This is even easier than checking out books, since you don't actually need to record who is attached to the account. Libraries could obfuscate information that could lead to invasion of privacy. And because it's distributed and local, it's much harder to abuse. If someone tried creating 100 accounts, the librarians would start remembering them and refusing to give them more. Going around to 100 libraries would also be much harder than spamming a new account page on some website.