I would not recommend memorizing 40 alphanumeric characters. Not only do I think it's overkill, but it's also dangerous because you could easily forget it.
You would be better off memorizing 12 random words, which is plenty secure and easier to remember.
Good idea.
Government Agent: "Please tell me your brain wallet password!!!"
Me: "I'm sorry, I forgot! It was 64 characters."
Actually, my use case for stuff like this is to have the characters printed out on paper. It's not a "brain" wallet anymore though.