I think you just need to use each object's rotation to rotate the pivot. Something like this.
for each(var entity:IEntityModel in this.collection) {
var dx:Number = this._x - entity.x;
var dy:Number = this._y - entity.y;
var c:Number = Math.cos(entity.rotation);
var s:Number = Math.sin(entity.rotation);
entity.pivotX = c * dx + s * dy;
entity.pivotY = -s * dx + c * dy;
}
The numbers c and s could be gotten from the transformationMatrix, but I think this is simpler. I am also not 100% sure which way things need to rotate. If it looks close but is wrong then changing the last two lines to the following might fix it:
entity.pivotX = c * dx - s * dy;
entity.pivotY = s * dx + c * dy;