Personal cameos are pretty rare in the anime world. To the best of my knowledge, this is the only time Isao Takahata appears in one of his movies. In this scene from Grave of the Fireflies, he has a very short cameo as a doctor's patient. He does not have any lines, and is only on the screen for a few seconds.
Now, the next question I'm sure you're curious about is, "Does Miyazaki make any cameos in his work?" The answer is, yes. Twice, by my counting. I'll post the pics of those later.