One minus I see is that the user has to store safely a string like AAwCnwGIAe0EWABWAI4AkAMjAFQBLgZjB1T1PJtz too. This is not that far from safekeeping a private key and you risk - then your trezor breaks (I didn't forget the use case) - that you don't remember what this AAwCnwGIAe0EWABWAI4AkAMjAFQBLgZjB1T1PJtz was for.

One plus is that one can safely keep the new seed at hand because it will not work with Trezor, Electrum or whatever.

I also have to say something about the word lists:

1. Keep one word list. If somebody will try to use a non-standard 65535 word list and will forget after some years what he used you may have some odd sort of bug reports there.
2. Keep one country. People will forget they've used this or that. Even worse, some will use incorrect files and will lose diacritics or other special characters and won't remember what they have to do to fix.

My mother tongue is not English, but on computers I stick to English because it reduce the chances for headaches.

You can take it one step further and convert both one-time pad and the resulting ciphertext into the same mnemonic format as the original seed. This will allow users to enjoy the benefits of mnemonic format - ability to memorize it, being easy to write down on paper and easy to transfer from paper back to computer.

It actually wouldn't make any difference, since decryption is just addition modulo 2048 and both parts are of the same length, so ciphertext + key and key + ciphertext both yield the plaintext.

Also, the cool side effect is that you can use both the key and the ciphertext as some sort of decoy wallets. Providing you have copies and backups, you can give them up to physical attackers to lose only a fraction of your coins.

That's true, but it also depends on how you encode the values.

I'm not sure it would work as decoy keys, because there is actually a checksum built into the BIP39. When you run the keys through the OTP it breaks the checksum. Unless you have a wallet that ignores the checksum, the words wouldn't work as a proper key (or at least, it's extremely unlikely they would work).