You could make an algo that takes identifier = left(word,4) and then locate(identifier) in an index of real words, but that's additional software, and it doesn't add any real security, except perhaps buy you 5 more minutes for someone that stole your seed phrase to figure it out and then manually map it against the standard.
If those few extra minutes are significant in your security setup, you may need to rethink your setup.
Most of these change BIP39 "ideas" aren't worth the effort. If you're not rich, your main enemy is loss. If you're rich, your main enemy is centralization. Neither will be solved by encoding tricks.
identifier = left(word,4)and thenlocate(identifier)in an index of real words, but that's additional software, and it doesn't add any real security, except perhaps buy you 5 more minutes for someone that stole your seed phrase to figure it out and then manually map it against the standard.