...
Just generate a giant random alphanumber (at least 256 bits to provide maximum entropy) and store it somewhere....
Of course that makes it more secure, but your wallet is no longer a "brain" wallet. If you are going to write down some 256 bit random number, you might as well just write down the private key.
The most effective way to make a brain wallet more secure is to use a different hash algorithm. SHA-256 is designed to be fast and is not suitable. Use one specifically designed for hashing passwords, such as bcrypt.
Exactly. The kind of wallet described in the OP would likely be less secure then a brain wallet that is sufficiently unique. An attacker could potentially find the copy of the "key" that you use with your passphraise and either destroy it or hold it hostage pending you giving them money to allow you to access it again. Also it would encourage someone to use a weak passphraise could find the private key with a small number of "guesses"