is it so hard to implement establishment of bases at foreign countries ? this kind of thing. Especially for cold war era and modern scenarios.
I think this could be done, perhaps using the Colony management rules for military facilities. You can build them in a colony and the hex becomes yours. What I think is not common knowledge, is, you can scrap/destroy the military facility and the hex will still belong to you. You may then build civilian facilities there in the colony and you maintain control over them, unlike if you simply build civilian facilities in the first place.
Perhaps, the core mechanic or some of the code which allows this could be tied to a treaty enabling one to build military facilities in another country. It seems logical if the treaty was broken or war declared between the two powers, the military facility would revert to the host nations control. To prevent the human player from creating a treaty and then packing many units in a Barracks with the intent to declare war and steam roll the host nation, the units become property of the host if at war with the facility owner.
As we say here, I am just spit balling, first thing that came to mind.