One thing that I think could be a neat addition to the customization would be 3 things: Battle Shirts, Vests and buttons.
I know that many confederates wore something called "battle shirts". Now I don't know if they would've been worn during the Maryland campaign, but I believe it could be something to look into. This was also a Confederate thing only I believe.
I also know that vests were worn during the civil war, and this could be a customization option for cavalry to wear on the field. Again, I'm not sure how common vest would've been during this campaign.
On final thing I believe would be an excellent addition to the soldier customization would be how many buttons are closed. In reenacting, we are told when its hot to unbutton some of our bottom buttons, and this could be a small, but neat addition to the soldier customization.