I think the Masterminds quiz is a great idea, but reading the request from this new forum member makes me wonder if it is getting in the way of one of the main points of the forum - helping people learn about the Beatles.
I might be a bit chuffed if I came to a forum with a question like theirs and was told essentially - "I know the answer, but I won't tell you now because blah blah blah". You see, a new guy (maybe not this enlightened soul, but certainly some others) might not understand why the reason for not helping them out now is so important, and all they will read/hear is "blah, blah, blah".
Maybe give the poster an answer to his question and change the Mastermind question? There are other options, like PMng the poster with their agreement not to answer that specific Mastermind question, but that doesn't help out the next guy who might have the same question ... seems a little bass ackward, doesn't it?