Can I make a locked iTunes 'house account' for a library?
In a nutshell: No, this is not possible without custom modifications, like with the display models in Apple Stores. It'd be better to set up the apps and content you desire, and then lock out the functions through the parental controls to prohibit any foul play. The last thing you need is legal issues, which validly could happen. App Store apps can be downloaded after purchase by simply authenticating on a separate device. Music and other entertainment forms cannot, and hence will be stuck on the iPad if a customer decides to buy them on the iPad, leaving you guys on the hook for any transaction problems that might occur.

Tread carefully.