The new ACUX shows this link "(help me choose)" prominently on top of the first textfield. This is a wikipedism that has several problems:
- Most MediaWiki instances don't have any username policy. With that link enforced you need to create a wiki page not to show that link in red. See for instance http://www.mediawiki.org/w/index.php?title=Project:Username_policy&action=edit&redlink=1
- Even if you do have a policy (like English Wikipedia has) this is probably not a page you want all new user to look, scared that they might be doing something wrong. http://en.wikipedia.org/wiki/Wikipedia:Username_policy is so dense and boring that it might drive potential new users away.
I'm not sure how to force a wrong username and I don't want to create fake accounts trying to test this, but I bet when you try to create a "blacklisted" user name you get an error message. Place the "(help me choose)" link in the error message instead, since that is the place where legitimate users will need it.
In the meantime most people just trying to register with their NameSurnameXYZ and other innocent variants will just go through, unaware (for good!) of the dense boring policy they just followed unconsciously.