I don't know that a statistical breakdown will ever be available, but I suspect a lot of Southern Baptists were present at the New Baptist Covenant. Indeed, many CBF churches in the South are dual SBC/CBF congregations. And many, many Southern Baptists have no interest in the direction the fundamentalist denominational leadership is moving.
An article by the New York Times, while not the final word, offers an indirect but intriguing take on the Covenant meeting: of four persons quoted, one is a young women with no stated affiliation; one is a layman in a National Baptist Convention (African-American) congregation; and the remaining two are Southern Baptist ministers - both of whom spoke glowingly of the Covenant meeting. There was no mention of either the Cooperative Baptist Fellowship or American Baptist Convention.
Many Southern Baptists have remained on the sidelines while theological fundamentalists have taken over and restructured the SBC in their own image. I wonder if issues such as racial unity, environmental concerns, and healing/social ministries might be enough to finally move some of these disenfranchised-yet-still-Southern-Baptists from the SBC side of the Baptist ledger to the broader, larger world of Baptists as represented by the New Baptist Covenant Celebration?