I think this entropy grid idea is a bad one because it usually is going to be words, or very common pixel patterns.

With a strong Argon2 parameter for the password expansion hashing, and about 8 words, you are still unlikely to see any break of the key in any sane amount of time.

For the border crossing situation, you don't want to have anything on you that would hint you are carrying a key in your memory. I'd suggest that using a common text as a codebook would make this a lot easier. Especially books like a given edition of the bible or the official, supposedly unmodified text in original arabic of the Qu'ran are both good options because you can get them from anywhere to refresh the memory and thus have nothing physical to signify you might be transporting a wallet.

Kinda cool in a way, I mean, this is a little like the concept of Mnemonic Courier, the basis of the Johnny Mnemonic short story/movie.