Even if koans like this have some legitimate insight buried in them, I wonder if the signal-to-noise ratio is too high to reliably extract that insight. Accurate, rational communication seems to be at least as hard as internal rationality.
I therefore propose an exercise of "reckless interpretation": Read the koan at a very surface level, then generate an arbitrary interpretation. Don't worry about it being too simple, too silly, or missing the point: The point is to miss the point. If these interpretations cannot be easily refuted with a technique other than "no, he meant it this way", then the koan is probably too ambiguous to serve as a useful communication device.
Here's mine for this one: Joshu is not teaching, he simply dislikes dirty dishes. The monk mistakes this for a legitimate insight, and by that unintended insight, the monk is enlightened.
A few reckless interpretations of mine: Joshu was able to determine what the monk ate, what type of dish he ate it from and whether he had washed it. This made the monk realize that his life was highly predictable, and he needed to vary it more. Or it made the monk realize that Joshu had Sherlock Holmes-style deductive capabilities. Or it made the monk realize that Joshu was telepathic. Alternatively, Joshu guessed incorrectly about the monk's diet and dishes, which made the monk realize that Joshu was no less susceptible to mistakes than anyone else.
Joshu Washes the Bowl
A monk told Joshu: `I have just entered the monastery. Please teach me.'
Joshu asked: `Have you eaten your rice porridge?'
The monk replied: `I have eaten.'
Joshu said: `Then you had better wash your bowl.'
At that moment the monk was enlightened.
Mumon's Comment: Joshu is the man who opens his mouth and shows his heart. I doubt if this monk really saw Joshu's heart. I hope he did not mistake the bell for a pitcher.
It is too clear and so it is hard to see.
A dunce once searched for fire with a lighted lantern.
Had he known what fire was,
He could have cooked his rice much sooner.