To make opponent strength unknown and have the "follow-on/Meeting Engagement" feel of a Civil War battle, perhaps you should just make liberal use of Reserves style rules for deployment? That would be the "simplest" way but not necessarily the best way.
Some games to look at that maybe useful around how to get this "escalating" engagment feel could be:
1. Chain of Command- They sort of wrote the book on this. The Patrol phase allows you to place "jump off points where units actually deploy from. Very innovative and would fit the feel nicely.
2. Jovian Chronicles- Beta Test Rules- Engagment forces tend to be split into groups and enter the board as individual elements at different points, with few or none coming on together.
3. Battlegroup Series- They have specific rules for Recon elments at the beginning of the game, and how having these elements benefit your later reserve roles, deployment options, and cause soem ill-effects to your opponents.
4. Blucher- They have a pretty good "blinds" system using cards and when a card is revealed.
These might all give you some ideas about how to approach it. I also recommend you check out Johnny Reb III, Longstreet, and Across a Distant Field as resources to consult for pretty solid/popular Civil War rules.
Sadly, I do not have any written reviews to point you at for any of these games. :(