I've heard this rumor for years, I suspect it is an 'urban legend'.
If it did occur, it would have been around the GM centenary (1960).
I can't imagine a 'pop' show having this sort of guest in which 99%
of the viewers wouldn't have had a clue who she was, or who Mahler was.
I heard she spoke English with a VERY thick German accent, so that
may have been a problem.