An intriguing suggestion and one which I partially agree with and I'll tell you why.

Yes, I wholeheartly agree with SL's being able to have the ability to delete double posts and to lock off a story once it has reached it's completion.

What I disagree with is the ability to alter people's posts.  Yes it is very easy for troopers to take a wrong direction in a story, but as SL it should be within their powers to either discuss this with the trooper and have them amend it or to put up a follow up post to steer the squad back into the direction that the story is to follow.  Troopers spend a lot of time and hard work in creating good quality story posts and for an SL to come along and edit it may demoralise that trooper and cause them to question why they spent their time to create the post in the first place.  A simple ComNet message or IRC chat with them to advise them that they've done something wrong is a simple thing to do and it lets them edit it in their own style.

I know this from experience when Paladin squad went down a wrong route with a story, a simple chat on IRC and we were able to edit it so that we followed the story's direction.