I think we can officially say that Willie Nelson has worked with absolutely everyone in his long career.
A while back I came across a blurb somewhere that mentioned that Willie Nelson had once recorded a song with Sinead O'Connor. You may remember Sinead as the bald Irish chick who pissed off Catholics by trash talking the Pope and ripping his picture up while performing on Saturday Night Live. She had that addictive song Nobody Compares 2 U back in the 90s.
Despite all her craziness, Sinead has a very interesting voice so after reading that she and Willie had once recorded a song together, I had to go find the proof. I ended up finding the official video for Don't Give Up, a song from Willie's 1993 "Across the Borderline" album.