While they didn't know exact positions they did have quite a bit of information. At Shiloh AJ had cavalry scouting the Union positions for some days before the attack. Grant was the one who failed to recon. At Gettysburg Meade know the position of every Confederate division on the day before so had a pretty good idea of when they would arrive. Lee however was blind as a bat.
The problem in our scenarios is its hard to reproduce this. Random reinforcements can skew a scenario badly so isn't the best solution. Not peeking gives a decided advantage to the player who has played the scenario before. They already know when things show up. While some of the most fun games I have had have been blind scenarios I have played to long to have this occur in any historic scenario. Sometimes in a campaign it is possible but even those are usually repetitive after a while.
It would be nice if the Campaign engine was a bit more sophisticated and could alter reinforcement schedules and entry points based on choices made grand tactically but it probably won't happen. Maybe Hank will pull his "Fight the War" together and give us a campaign engine that will do this. He keeps letting life get in the way.
Shame
