Great idea. How about:
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Surely, no one will ever remove those Bitcoins from their address again. Not even me.
Seriously, it's easy to find something as secure as your stuff, Thomas, or even more so: Just find 2 sentences that relate to you and only you. Then add some garbage, such as spelling errors. Or replace one word of your sentence by it's 17031988*md5sum, where 17.03.1988 is your birthdate. Or come up with some other source of noise that's easy to remember for you, but impossible to guess for a cracker.
Tell me, what's your goal, exactly?
I guess you are not requesting a new feature, because it's already possible to do what you say with the current client.
So, perhaps you want me to say "yes, users can derive their seed themselves, and this method is officially approved by the developers"? Is that what you want? or are you just a troll?
I did not say that you cannot derive a seed that can resist an attack. But I hope that you will agree with me that there exists a fraction of users that are not as smart as you are. Some of these users could, by following your principles, end up being robbed. Do they deserve to lose their money just because they are not as smart as you?