This would only be effective if you know that a leak is going to occur. Otherwise, this wouldn't mitigate any specific leak as it is unknown when the leak would occur.

However, a strategy that could worsen the effectiveness of Wikileaks would be to constantly spam them with large amounts of plausibly authentic fake documents. Maybe I'm missing something but this seems like such an easy way to tie up the resources of Wikileaks that you'd have to wonder why these governments and corporations aren't already doing this.