I think you have it exactly backwards: the tape is win-win for supporters of the war effort. If it shows a pro-war Berg, then it undermines his father's hysterical finger pointing and makes him a martyr. If it shows an anti-war Berg, then it opens the door for questions about how he came into contact with the terrorists (and also demonstrates that he was about as wrong as could be in the process). The publicity fat boy gets from this-let's face it his audience is limited to lefty true believers and the French-is small potatoes compared to how the good guys stand to profit.
Yes, unless the actual contents of the tape are never really revealed, and the only publicity winds up being the
"rumors" that the Administration or its envoys are hot on the trail of Moore in an attempt to get something we're supposed to think would be damaging to them. Much politics could be played with this tape, all of it Moore's and all of it to serve direct political purposes of the dump Bush crowd. In other words, I don't think the film will ever be seen. "The Medium is the MEssage".