I recently watched the Magnum PI episode "Past Tense" that aired in 1982. It's the 4th episode of the 3rd season and it felt like it was written to include Jack Lord and Five-O.
TC's helicopter is skyjacked and he is forced to land in a local prison and help a prisoner escape. Following the escape, TC and Higgins (who was in the helicopter when it was hijacked) are taken hostage so the prisoner can get the attention of Magnum to finish him off. Magnum works with HPD Lieutenant Yoshi Tanaka (played by Kwan Hi Lim) for clues.
It just seems that a prison break of this magnitude would have been a Five-O matter. And 1982 would have been a perfect time to have a McGarrett cameo as the Magnum series was firmly established by that point. (Meaning it would not have felt forced like some series will do when they're struggling in the ratings.)
I've read rumors that Jack Lord was invited to appear on Magnum, but declined. Does anyone have any additional information about a possible cameo?