But back to the game, I saw an idea mentioned elsewhere that I really liked. Could you have the ACW much like the Spanish Civil War in HOI3, and have an option when the states secede as to whether you would like to be the Union or Confederacy? I wouldn't mind playing 25 years as the US if this was so.
Save. Load. Select.
Doesn't seem too hard, although you're right it's more straightforward to do it within the game if the engine can support it.