I felt idiotic for not knowing that Sinatra had recorded this, so when I sat in the audience for the LA production of MERRILY last year, I was super surprised to hear it. At first, I was like... "Damn! That is a GREAT Sinatra impersonation!" And then I was like "Damn! That has got to be the real Sinatra!"
I'm sure Sondheim would talk about it if someone asked.
-- Also, re: Judy Collins, I will concede that "Send In The Clowns" is a good example of a radio hit from Broadway post "Age of Aquarius."
But I think my main point still holds: pretty much no one does this.