I heard about this a couple years ago when my orchestra (I'm a violinist) did a "movie music" concert with a guest conductor (Hal Beckett) who worked in the industry. We opened the concert with the fanfare from Also Sprach Zarathustra... then he turned to the audience and explained about temp music, and we played the Alex North fanfare.
I have a feeling that these sorts of stories fall into the category of "well known in the industry but not thought to be interesting enough to write down or relate to outsiders".
In this case the story was printed in a booklet accompanying a recording of the score, though I'm fairly certain it had been reported on before that too:
I have a feeling that these sorts of stories fall into the category of "well known in the industry but not thought to be interesting enough to write down or relate to outsiders".