As has been firmly established by the unheard voices that have throughly deconstructed Bowling For Columbine, Moore's girth is only exceeded by his propensity for deception. He's gained access to a lot of people by being deceptive about his motivation for interviews.
For example, on his short-lived NBC show TV Nation, the beauteous Karen Duffy interviewed insurance company executives who pioneered a practice which gave terminally ill AIDS patients a percentage of money their beneficiaries would have received so they could travel or spend in the waning years before a premature death. Because this saved the insurance companies money, the companies were not shy to potential investors about how this improved their bottom line.
In the final cut, Duffy's "report" showed grinning marketing executives who were profiting from the suffering of people dying of AIDS.
Conspiratorially, I think that the footage (if it exists) shows a very anti-war Nick Berg.
Remember the way that Moore made Charlton Heston sound like he was insenstitive to the feelings of the parents of Columbine victims by inserting excerpts from other Heston speeches out of context?
My guess is that had he survived, Moore's Nick Berg footage would have been carefully edited so that a pro-war Berg would sound like an absolute moron. However, after becoming an international symbol of the bloodlust of the figures targeted in the Iraq invasion, it would screw up Moore's narrative to have an ostensibly humanitarian Jew end up beheaded by Al-Qaeda in the middle of a Bush-bashing film.
I also think that Moore is making sure that whatever footage he has is only used to further the cause of Michael Berg's antiwar crusade. Michael Berg has been making a lot of disconnected allegations suggesting that the USA is responsible for his son's kidnapping and execution. You can be sure that only footage that will back that up will ever be revealed, and that all that wouldn't ended up on the famous "cutting room floor."
Interesting theory. The problem for Moore is that he does not "do" unedited film and that's all that people would care about. But why would Moore announce it if it makes him look bad (or Nick look good)?