Ironically, stating this at the beginning of telegram would precisely cause what it seeks to prevent (vulnerability to known plaintext attacks).

Which makes me wonder: how many permutations of this rule could be conceived (and needed) that on the one hand would keep the point clear to the receiver, but on the other hand prevent such attacks?

In any case the best option is to not have (to repeat) this rule inside messages.

It could be sent in the clear, although since the point was to apply it to every encrypted message, that would likely already have been redundant with having originally been encrypted. Just consider it part of the decryption algorithm itself instead: step 1, attach warning text, step 2, initialize decryption state and decrypt.