Resurrecting a very old thread, I think this story could well be true. Mitchum didn't make his first movie until 1942, but in the years before that, he had been trying to make a career as a young writer for hire around town, since he'd come to live in LA in 1937. He started acting first with the Long Beach Players Guild, and wrote a few plays for them, and around the same period was selling material, skits and songs (apparently risqué) to nightclub acts, and had some work supplying dialogue for radio serials (his sister, Julie aka Annette, was acting in one).
The comedian Benny Rubin became aware of his nightclub work, and the two struck up a deal for Mitchum to write material for Rubin's act. The story goes that Rubin was invited to perform a short piece as part of a 1939 German Jewish Relief Fund gala show staged at The Hollywood Bowl, and that Mitchum wrote the oratorio Rubin performed on the night.
Now, I've not been able to find much of anything much yet about this particular gala night. But given the whirl of activity Welles was engaged in around 1939-40, and that he was just beginning to be courted and introduced around Hollywood, and that this is exactly the sort of thing he would have got involved with, I wouldn't discount this story out of hand, especially given the Welles-Rubin/ Rubin-Mitchum connection: Welles used Rubin a few times on radio around this period, and would later cast him in Kane, and The Other Side Of The Wind.
It would be great if someone could turn up more info on this event. If you check out this newspaper archive link, by the way, you'll find a column entitled "What They Once Were," compiled for Walter Winchell by Rubin, where he lists Mitchum's previous career as "songwriter."http://news.google.com/newspapers?nid=1 ... 53,1989722