What do you exactly mean by "brain damage"? I have never really understood this word. If it means that kava, the root, maybe could cause physical wounds in the brain, well then there is only one solid answer: no of course it cannot do this - and why should it? There would be no logical or scientific reason for this.
If a substance has a "bad impact on the brain", I think we should be careful about using the word "brain damage". It sounds very horrible and physical - like some guy cut a sword right through the brain. When talking about different drugs impact on the brain, I think it is important to remember how different people are responding to different substances. For instance, if a person has an anxiety disorder that has been triggered by a drug-experience, I would not call it "brain damage". I would rather say that a specific traumatic experience have had an impact on the individual and his/hers nerve system, subconsciousness etc. which has roots in the persons state of mind etc. etc. Brain damage as an objective fact from consuming a specific substance is nonsense IMO