I don't think that a break in communion requires an ecumenical council. Even in the time of the councils, breaks in communion happened on a more local or regional level without the "sanction" of an ecumenical council.
But, even so, aren't all Orthodox anathematized by the terms of the declaration of the First Vatican Council?
On second thought, do you mean that those who didn't agree with the terms of Vatican I were anathematized? I suspect you're probably right there, though I haven't looked at Vatican I in awhile.
But the eastern churches wouldn't call Vatican I an ecumenical council, would they? I know we in the western churches do, because we use that term for every council that involves all bishops in communion with the Pope. Regardless, no one was excommunicated at Vatican I. Accomplishing that would have required a specific act.
Now, what I am proposing here is that excommunications of bishops must follow the procedure set forth in the Gospel According to Matthew:
"If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven." (Matthew 18:15-18)
Jesus is talking to the twelve, here, which is what Matthew means by "disciples." Hence, this applies to their successors, the bishops. It appears, then, that the way that Jesus authorized the excommunication of bishops was by a decision of the whole Church, which requires a conference of all the bishops in apostolic succession. To the extent that individual bishops have tried to excommunicate other bishops by themselves, it doesn't pass muster with the procedure that Jesus, who should have something to say about it, laid down.
Now it is true that the passage I just quoted is followed by this:
"Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I in the midst of them." (Matthew 18:19-20)
But it is clear that Jesus is moved on to heavenly ratification of prayer requests of the apostles and their successors, and that this last passage has nothing to do with letting someone be to the Church "as a Gentile and a tax collector."
Brendan, the past is fixed, and we can do nothing to change it. We can only move forward. If unification is a desirable goal, we must posture ourselves in this way. If we fixate on the rancor of the past, we will never get past it.