What is the legality of an American tax supported public library claiming IP rights on materials created by employees on site, outside of assigned tasks? I understand the claim of materials assigned to be made, such as curricula, presentations and publications, but what about professional creations, such as conference presentations, process development, or other professionally creative works?
IANAL, IANYL, but here's what I can tell you.
Public librarians are not direct federal employees, so works they create are not subject to the usual automatic public-domaining of otherwise-copyrightable documents created by federal employees.
The 1976 Copyright Act contains the notion of "work [made] for hire:"
The tax-supported status of the employer is, as best I can tell, irrelevant; if the creation of the work is within the scope of employment, it's WFH.
Whether "on site, outside of assigned tasks" is "within the scope of employment" would take a real lawyer (and possibly a court case) to untangle, I suspect. To protect IP rights to one's works, it is best to create them on one's own time and away from the workplace.
As I understand it one's employer may claim the IP to anything you create while on the clock. Things which you create on your own time, even if you use them as part of your professional duties, are your own IP. The kinds of things you're talking about seem like the IP was created elsewhere but the tools you're presenting it with were created at work thus your employer could claim ownership of the tools you use to present the IP, which are a kind of IP themselves, but not the information contained within them which you created on your own.