There's no way to make a territory off-limits to only one side?
Pity.
For example, in the case of foreign intervention, I could see making the UK and France off-limits to USA troops but still open to CSA. Otherwise you might get some really odd outcomes if the USA manages to land a force and actually conquer Britain or France. But you want CSA ships to be able to take refuge there, as well as the Brit and French expeditionary forces (which must be treated as CSA units, I'm guessing?).