I've seen someone (I think ryanc) mention before using a combination of a passphrase plus a random (weakish) seed. The seed needs to be printed out and stored somewhere safely. The beauty of this arrangement is that the seed is weak enough to be expendable, but strong enough to add some extra protection against casual hunting. If the seed is lost, you can use a program to brute force it until it finds a match for your brainwallet address. The strength of the seed is chosen so that some time (say one to two days) of brute forcing would be required.
Interesting idea, while it's far less secure than CSPRNG/PRNG, it's acceptable assuming no one know you use this method.
But we are started talking about brain wallets and brain wallet feed could be generated randomly.
I don't see the point of using random passphrase on brain wallet, unless you try to remember it. Which means either you use short passphrase or you could forget it easily.
I can keep in mind 16 random generated words, but problem that words are already existed and could be generated again. Good way to change 1 word from this 16 to your own created word.
Don't forget dictionary attack