Buffett is always pretending to be the non-activist investor, but I think that he can exert much power behind the scenes (assuming he does not do this already, this certainly warrants it).  With 10% of WFC float, he certainly could directly lobby BoD members without it being public.  

As a major shareholder, it is his responsibility to do so. If he is not, then he is implicitly agreeing with the rotten WFC culture.