Who said anything about "random alphabet-number-symbol mix"? Alphabet-number-symbol mix is terrible cargo cult security and shouldn't be used. Go reread my posts. Good schemes for memorization use a mnemonic encoding that translates large integers to and from english text.

Though generally the properties that make phrases _easy_ to memorize are what actually produce the statistical properties that make them easy to predict.  But predictability is resolved by using a lossless encoding process with known entropy rather than a human source.

(And memorization still has a rather severe forgetting problem. Like people overestimate their ability to be random, we also over estimate the integrity of our memory. The public has very little experience handling keys which cannot ever be recovered if lost.)

I actually have IRC logs about the creation of the phrase brainwallet and brainwallet.org.  It was created by someone who introduction to the subject matter was his own efforts to crack peoples insecure keys, and he was irritated that he only found a few coins. No kidding. Don't try to glorify history to people who watched it first hand.

It's possible to memorize keys without using an insecure generation scheme. Though if you think thats going to protect you from evidence of unlawful activity on your computer: (1) Perhaps you should engage in less unlawful activity.  (2) you're going to be rather disappointed when you realize how complete computer forensics is, hiding a key will probably be the least of your concerns.