Is there a confidentiality clause due to union regulations? Or does most of this knowledge just come from the rejected actors volunteering the information themselves?
It's usually a matter of luck that fans encounter this kind of information. Studios put out media releases when an actor wins a role; they don't usually promote those who turned up but failed the audition.
Sometimes the news does filter out, either in interviews or when actors are at conventions.
"The Making of Star Trek: The Motion Picture" has a reprint of a memo listing the actors called in to audition for Will Decker (one of whom was Andrew Robinson).