I could have sworn there was a movie called BARQUERO RIDES! (with an exclamation mark), which was a sequel to the original Barquero (1970), starring Ol' Snake Eyes. But I can't find it anywhere.

I know he did a sequel to SABATA(1871), and a sequel to THE MAGNIFICENT SEVEN called something like THE MAGNIFICENT SEVEN RIDES, but no BARQUERO sequel. Was I dreaming the whole time?

I think you were dreaming. Van Cleef's filmography does not show a sequel to Barquero. I'm guessing you dreamed you saw a Barquero sequel with the word "Ride" in the title, which you're now confusing with The Magnificent Seven Ride!

Or something like that.

I thought, "Way cool! Barquero gets to ride away from his old ferry-boat & kick some more Western butt ..." Maybe it was an idea someone had floated at some point which never saw the light of day. I'd have paid to watch it.