Why would you need to waste time and effort creating a method to encrypt a bitcoin private key when you already have plenty of perfectly adequate (and proven) encryption systems for encrypting "plain text"?
A bitcoin private key is not plain text, it is technically a number and can easily be treated as a stream of bytes with fixed length (32 bytes). A mnemonic on the other hand is technically plain text even though it can be converted to a stream of bytes but it will have variable size.
With that said I'm all for a "standard" way of encrypting a mnemonic, I've already posted about what I think such a method could look like a couple of times on this forum.