my previous post on Watchmen characters cameoing via signature in an episode of Teen Titans Go!, Watchmen character Nite Owl did one better and actually appeared on-scren for a split-second in the episode "Real Boy Adventures."
I was surprised to learn that, if only because I had just watched that episode within days of seeing the "Yearbook Madness" episode. If you haven't seen "Real Boy Adventures," and you should, the plot is this: Cyborg is disappointed that his mostly-robot body doesn't allow him to enjoy hot tubs, so Raven uses her magic to make him "a real boy" again. No longer a cyborg, he's forced to change his name from "Cyborg" to "Fleshy Guy," one of the many negative consequences to being a real boy.
Robin, a fellow real boy, tries to cheer him up by taking him on a "real boy adventure," which includes a musical number. That's the part where Nite Owl appears, chanting the chorus to the song along many other comics characters and historical figures. I didn't notice him the first time through, probably because I was so distracted by Batman's guitar solo after the lyrics ("You can dance all night in the rain real boy/Did you know Bruce Wayne is a real real boy?").
I'm fairly certain either Beast Boy or Robin also has a Nite Owl poster hanging in his bedroom, but I didn't check for screen-capping purposes.