Although (brain wallet produced by human) --> seed key
is recognized and accepted as a bad idea,
f(brain wallet produced by human) --> seed key
where f is a easily remembered math procedure such as modulo(x), may form an acceptable key

that's ridiculous as f() is also produced by human.
Not to mention that in order to get a 256 private key value from the memorable passphrase, you need some kind of f() anyway.
WTF does it even mean that something 'is recognized and accepted as a bad idea'?
Sounds to me like an argument brought by someone who has no arguments