Joseph Cotten is one of the great unsung screen actors, but after what happened with Ambersons, I doubt that Welles viewed him as a "good-luck charm."
Cotten's portrayal of sad Holly Martins in The Third Man anchors the picture and is as brilliant in its own way as Welles' electrifying performance as Harry Lime (but Cotten has the larger role and carries the picture). Cotten was equally effective as the serial killer in Hitchcock's Shadow of a Doubt, but (like Glenn Ford, another unsung screen giant) was never even nominated for an Academy Award.
Recently, I saw Cotten in Richard Lester's Petulia (1968), one of the few great roles he was given in the later phase of his career. Cotten didn't bring Lester any luck either, as Petulia tanked at the box office, despite the presence of stars Julie Christie and George C. Scott in their prime and cameos by Janis Joplin and The Grateful Dead. But Petulia has gained a certain stature over the years. Leonard Maltin said it was one of the best American films of the sixties.
Cotten does not appear in The Other Side of the Wind, according to IMDB. He is supposedly in the crowd scenes early on in Othello (standing beside Joan Fontaine). The two were in Italy shooting the romance 'September Affair' and made uncredited appearances in 'Othello' as a lark. I've never been able to make out either one of them.