Rarely hurts to ask, and you don't have to tell the whole truth.
Pick a library that you're interested in, go through and do a basic scout walk through with a sketch pad to get a rough idea. Possibly do some basic mockup shots with stand in props.
Play it off as planning a fairly conservative photo shoot with a focus on artistic value. Sell this plan to someone at the library, focus on that plan while shooting. Work with them to find a time when you will be the least disruptive, and if they agree to help you out with an arts project then you just happen to sneak in the more 'naughty' shots when no one is paying too close of attention. You could easily kill two birds with one stone by no only getting the shots you are looking for, but also providing photos to your local library for their own use. (Hopefully the two sets don't share much in common for poses, but I think the "naughty librarian" concept is better sold visually by someone who in the same afternoon could pose for photos to be used on the cover of a flyer. After all the whole image is about going from perfect/nice/proper to naughty.)
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500