Dont do this. Hashing a pass phrase will not result in additional entropy. If an adversary were to know that you hashed a brain wallet pass phrase, they could do the same to brain wallet candidate pass phrases they believe you are likely to use.
It does the same harm it does for an attacker to know that my password is 18 characters long. It lowers my security, but it's infeasible to break it if chosen properly. Same happens with more hashes; if an adversary knew that I'm hashing a million times, he'd need a million times his computational power to do the same work.