So, I can imagine a scenario where Yoda says cryptic statement X, and then privately expands on X to make it specific in a letter, and then a month later the person says "I finally get X," and then Yoda's letter is opened and checked.

I can imagine it too, just not with actual Yodas. (Well, Yoda is fictional, but you know what I mean.) I could even generalize this beyond Yoda: Most people won't help you figure out whether they're saying something meaningful or just spouting hot air. This is especially true for the ones that produce the most hot air--after all, they don't want you to know that they're doing it. Helping you know, by giving you sealed envelopes or anything else, would defeat the purpose of doing it.

Even the ones that produce hot air out of sincere ignorance probably don't want you to be able to figure out whether they are spouting hot air--anyone who did would get selected out of existence.