I hate to say it because he's a favorite, but for some reason I think it could be Booster Gold. His knowledge of the heroes could stem from his being from the future and his personality is affable enough. Plus, in his conversation with Plastic Man, everyone assumes that the Secret Six refers to the Tangent Plastic Man of another universe, but maybe there's a future Secret Six line-up yet to be introduced that Plas is a member of. Hell, maybe it's spinning out of this event, featuring the characters like Manhunter and Question that are currently being used here.

Of course, if it's Booster, the possibility of it being a future/different timeline variation of him exists so as to not ruin the Booster that's currently running around.