You might consider limiting this to individual option.
For example, having messages signed by the address used in their profile or something like that.
This ^^^
It can be semi-automated anyway.
It provides an authoritative identity validation which accomplishes your goal in part.
You can pull the userlist of BitcoinTalk already and reserve those usernames pending this sort of authorization for the other part.
And it avoids the SSO issues.
also the bitcointalk forum is not using OAuth so that avenue is not open anyhow.