I believe it has - on New Years Eve 1999! It is estimated that Epcot can hold approximately 100,000 people. Short of rigning in the new milennium, I doubt that's too likely. Keep in mind that they may close the parking lot or to guests arriving via car, then to non-resort guests or guests without ADR's etc. I don't know how all the "phases" work these days. If you are a resort guest with ADR though you'd be among the last excluded. To reach the point there a fire marshall is literally saying no more bodies - I think that's a longshot for Epcot.
Now, at the other parks it is possible, but Epcot has the largest guest accessible space.