...Okay, let's take this from the top.
1. There is an Animaniacs episode ("Taming of the Screwy", for the record) that has a rather heavy dose of Hollywood cameos that may not be entirely understood to the modern-day viewer.
2. The same goes for the 1941 Merrie Melodies short "Hollywood Steps Out".
3. The cameos in the Animaniacs episode come all in a bunch near the end of the episode.
4. The celebrities appear throughout the Merrie Melodies short and are the focus of its plot.
There, I think that covers all our bases.