It's an interesting problem. The Civ's AI seems to be intentionally limited in many ways to play like a human. That probably includes some kind of Fog of War for them aswell, otherwise why would they explore? In fact, have you given the Civ explorer units? Offensive and Defensive units aren't as likely to go out and explore the map.
From a LUA angle, using Tuner you can make the map visible to a Civ, and I believe their are lua functions that allow you to declare war. You may want to write some code at the start of the game to make the map visible to the hostile Civ, and declare war immediately with all other Civs. Combine this with the normal XML edits to change Civ behaviour.
You could also try using the Barbarian Civ as your invading force, as they appear to "know" where the other Civ's are. Problem here is that they will only pillage and not occupy cities, which I'm pretty sure isn't what you are after.
All of this is speculation I'm sorry. I'm only just beginning modding myself, so have spent the last few days digging around to try and get my head around what is/isn't possible.
