Back in the 1987 update of DC's Who's Who series - which was a glossary of all the DC characters post-Crisis on Infinite Earths - McFarlane did a wraparound cover for issue #4. He was on Infinity Inc. at the time, if I'm not mistaken. You can see the front section of that cover here:
Inside the issue was a write-up for The Minutemen, the Golden Age super-team seen in Watchmen. Here's that write-up:
Note at the end how they made a point to say the Watchmen Universe
was NOT a part of the DCU in the above scan
And there you have it. Todd McFarlane of Spawn and Spider-Man fame officially drew Watchmen art for DC.