You might consider limiting this to individual option.
For example, having messages signed by the address used in their profile or something like that. Single Sign-on and shared userid is not an advantage to security. Generally it is the opposite of that.
The only thing we are looking for is integration. Meaning when users sign into our site, it will sign them into bitcointalk.org. Our end will have nothing to do with it. Security will not change. It works just like the FB integration that you find in most forums/websites no a days.
In other words, a user on our site would click "Comment" and a box would pop up and say, "Would you like to log in using your bitcointalk.org ID ?"
I think I understand you correctly.