Unless he is famous and known to others as brownwallaby + his real name, there is no defamation.

You cannot defame a pseudonym.

Your post re Baz Luhrmann is also irrelevant. It relies on the simple test of reasonableness. Would a reasonable person know who I was referring to if I called him Baz?

Yes.

Would a reasonable person know an anonymous Internet user's real identity if I referred to him by his username? No.

There is no caselaw to support your flawed interpretation of the law.

No defamation here so tell it to a judge