As Bitcoin gains in popularity, what is more likely than an intentional cracking is that someone else (of the over 7 billion people on earth) that also happens to be a fan of both the game of chess and Fibonacci accdentally use the same sequence as you.

Humans just aren't very good at being random.  We just aren't wired that way.  We tend to share interests and thought patterns.

Even if the others that like chess and Fibonacci don't choose the exact same sequence as you, one of them very well may mention in a public forum that he has an idea for using chess matches and Fibonacci sequences to create a brainwallet.  At that time, a hacker can create a program to cycle through every published chess game applying Fibonacci sequences to the selection criteria.

As much as we all like to think that we are the only person capable of choosing a particular pattern for selecting information, the fact that a pattern is being used at all is a strong indication that someone else in the world somewhere would come up with the same idea eventually.