One bit of trivia I recall is that Brennan played a bicyclist attacked by The Invisible Man in the Universal picture of that name. Too far away to recognize, though.
Brennan always did seem to be old, though. Unlike Jimmy Durante, who also always seemed to be old, but you could actually find pictures and film clips of a young Durante with hair. I can’t recall ever seeing a young Brennan.
I have a recording of Brennan reading works by Mark Twain. It’s on a collection of Twain stories, read by various actors, and in the older ones Twain is definitely expected to be old – he’s invariably voiced as an old man, by older actors – which is undoubtedly why Brennan is reading the part. (People seem to forget or ignore that he started lecturing while still in his thirties. The modern trend is to present a younger Twain.)