Today I received an email from Donald Ziems, a student of Professor Lippold Haken. Turns out Haken's ECE 395 class at the University of Illinois Urbana-Champaign has a team of students restoring an ancient PLATO IV terminal as a class project (cool!). Donald pointed me to the class wiki where they're tracking the progress of the project, including a bunch of photos and a PDF of the "PLATO IV Operation Manual".