Titanic Honor and Glory has a free demo out which sort of follows that concept with the points of interest and ambient noise http://www.titanichg.com/ You could go all out and make ghost armies at some points.

Do you have any connections with the park service at Antietam? They're like Civil War nuns, one of them has worked there in the basement at the Visitor's Center 30 years. The Battlefield Trust is also pretty active in trying new stuff recently especially augmented reality and specifically at Antietam.