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.
any number can be encoded as a hexadecimal string. which you can think of as a text string composed of the characters 0-9,A-F. there is a 1-1 relationship between the hexadecimal text string and bitcoin private key that it represents. Or just think of 256-bit text string consisting of 0s and 1s. That's a string too. or it can be thought of as such without causing any confusion or problem.
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.
Very good! Maybe I can locate them somehow.