I remember reading somewhere, I think it was fluffypony mentioning one could use for example words from a letter to a loved one seed for a Monero wallet. But from playing with https://xmr.llcoins.net/addresstests.html I realized that it is not possible to enter just any text in the mnemonic seed; it just doesn't accept improperly formed mnemonics, apparently. And even if I had 24 words chosen from the official word list, I would still not know what checksum word to add at the end... (I haven't tried generating a brain wallet in monero-wallet-cli yet, but if there is a way, please let me know.) Is there a way to create one's own mnemonic?
On the other hand, I am assuming that the hexadecimal field does accept anything, since it is the private spend key. So maybe the way to go would be to choose one's own preferred passphrase, take SHA256 of the passphrase and enter it in field 2. Is that how it should be done?
Obviously, one should be very careful choosing such a passphrase, but I am thinking that brain wallets in Monero shouldn't be as dangerous as in Bitcoin simply due to the inherent time consuming operation of scanning the blockchain and trying to decode every single output for the corresponding stealth address. I am guessing that a decent computer should take at least a few seconds for each brain wallet, so that compiling a huge list of brain wallets and monitoring them is unfeasible. Is this understanding correct?