Possibly, but IMHO about the wrong things. The weakest link in your reasoning is your memory. Many things could happen yo you (short of death itself) that compromise your memory. A mere blow to the head could suffice to cause sufficient brain damage to render your memory unreliable.

Brainwallets sound like a nice easy concept, but it is very hard to do this properly. DannyHamilton has given very good advice upthread. You really need to do the research to understand why this is so.

For example you have commented several times that you can do an offline transaction to transfer bitcoin without exposing your public keys. This shows ignorance of the workings of the bitcoin transaction mechanism. You have to broadcast that offline transaction to the network for it to take effect. At that point you have also exposed your public key since its an integral part of the transaction.

If you do decide to use brainwallet.org you need not worry about the website breaking down or becoming unavailable. That particular brainwallet simply uses a single sha256 hash of the passphrase to generate the private key. Any competent programmer can replicate that for you. But unless your awesome brainwallet scheme includes at least 192 bits of truly random entropy it will be less secure than a key generated by bitcoin-qt itself.

My advice. Use an officially supported wallet. Choose a good passphrase, write it down and lock it away in a safe or perhaps give it to your lawyers for safekeeping (being sure to advise them not to copy or expose it). Backup your wallet and keep copies in several safe places. Your biggest risk is relying on your memory alone.