Whether or not they're a part of a different "universe" doesn't really mean they still can't be a part of the show. They're still a cartoon in one sense and they can lack the whole "breaking the fourth wall" thing.
They can definitely put them in as cameos in the least, maybe even throw in some of the Animaniacs characters if the WB still owns them.
They can definitely put them in as cameos in the least, maybe even throw in some of the Animaniacs characters if the WB still owns them.