The phrase "a passing footnote" is correct and usable in written English.
It can be used to refer to something that is of minor importance or a brief mention in a larger context.
Example: "The author's comments on the subject were merely a passing footnote in the broader discussion of the topic."
Alternatives: "a brief mention" or "a minor detail".
And in this election cycle, while jobs and economic growth are supposed to be the key triggers for this election, the Internet and broadband get nothing more than a passing mention, a footnote more than a headline.
FOOTNOTE -- On the LCD (lowest common denominator) scale, the film rates a 9, saved from a 10 by a passing attempt at wit now and then.
David Wise reports the complacent cynicism of the Russian spies, but only in passing, as an amusing footnote to a tale of monstrous treachery.
Apart from a general discussion in his Idea of Principle in Leibnitz and the Evolution of Deductive Theory and an essay, "Martin Heidegger and Language of the Philosophers," Ortega referred to Heidegger only in passing in a long footnote in his essay, "Goethe from Within" (published in 1932) (Obras, 8: 271 84, 296 300; Universitas 7, n.9 (1952): 897 903; Obras, 4: 403,541; 9: 625–44).
How this country has changed, I thought: the son of the President is now an ordinary snitch, a man doomed to look over his shoulder for six one-hour episodes, and then die invisibly, his passing just a footnote to a larger drama that has little to do with him.
Once a classic, now a footnote.
Include a lecture in a footnote source.
America might not be grown up enough to handle it, but hopefully, before another 80 years passes, Christopher Columbus will be a footnote to Indigenous People's Day (and not the other way around).
Passed by, in every sense, it lingers as a footnote on a footnote of history.
A clever bid for a footnote in posterity.
