But you'll also have a real hard time convincing a court you have a free speech right to uppercasing and lowercasing letters in certain ways - the speech is in the words, not the presentation. That is, if there's a law banning encryption as a whole (and not simply certain messages, encrypted or encrypted), I think this still doesn't save you.

I think the courts have generally held that you can't simply throw speech onto some otherwise prohibited activity to gain First Amendment protections. For instance, see the case about the man who tweeted a GIF at Kurt Eichenwald with the intention of provoking a seizure - the fact that the GIF also contained words (... which were "you deserve a seizure for your posts") seems unlikely to say that the act of sending the GIF itself was protected speech.

Good: if the courts don't identify the decoding bible chapter, you get plausible deniability of "I didn't communicate a message for Illegal Activity X, I was just sharing some abstract ideas on theology with a friend".

Bad: Courts might begin issuing surveillance warrants (or worse) to TLAs based on the most innocuous personal communications, because "we're pretty sure that happy birthday e-card contains an encrypted message".

In either case, I completely agree if the message is deciphered by authorities, they've got you dead to rights, whether it's done by PGP, GPT bible verses, or yellow socks on Tuesday.

EDIT: I was commenting primarily on the GPT technique, but now having read the Memespeech page, totally agreed that it wouldn't even slightly stand up to scrutiny in court, as one needs no prior knowledge to decode the message and demonstrate that there is no other legitimate explanation for the communication.