Finally threw up my hands and created a Rao entry at Supermanica:
http://superman.nu/wiki/index.php/RaoJust an initial place-holder entry. I need more info.
For instance, when is the first time Supes refers to Rao?
The Answer Man himself, Bob Rozakis, notes that:
It was decided by editor Julie Schwartz to have Superman use the exclamation instead of "Great Scott!"
http://www.silverbulletcomicbooks.com/bobro/10660841129004.htmso it must be sometime around 1970, no? I am not aware of any actual Superman or related family member quote about Rao (as opposed to a "Great Rao!"-type oath) before the stories in Krypton Chronicles, etc. But my collection has many huge huge gaps.
Anyone?