I'm about ready to add a "guerrilla recruitment" module to the game. Here's how I have it working at the moment, so let me know if you think there should be changes, or whatnot.
How it works:
Between turns, tiles are checked to see if any 'guerrillas' should be given extra units.
A tile qualifies if it has a fortified "revolutionary" or "nationalist" (unit with Rev. or Nat. in name). Barb nationalist/revolutionary units qualify even if not fortified. The "guerrilla" tribe must not have a city within 3 squares of the tile in question.
Each tile is given a "value", which is calculated in the following way:
Find the nearest city (as the crow flies, i.e. not taking terrain or water into account), and add up the raw food, trade, and shields that tile would produce for that city (taking into account technologies, city improvements, etc). (The nearest city can still be owned by the guerrilla tribe.) Subtract 2 times the distance in tiles to the nearest city.
If the tile is developing or devastation, it has a value of 8 regardless of how far it is from the nearest city. If the tile is industry, its value is the production total, regardless of the distance to the nearest city.
If the tile is directly adjacent to the nearest city, the value gets +4 (even for the special terrains).
There is a chance of 4 units being created: the revolutionary/nationalist type occupying the tile, a gun truck, an RPG, a field artillery. The chance for each unit is tileValue/(3*productionCost).
I figure that requiring the "guerrillas" to be at least 3 squares away from their own cities will mean that you can't recruit guerrillas from your own territory. The exception is that it might make sense to have a presence in the villages in the backwater areas of South America or Africa, or maybe to keep a close watch on some natural resource.
As for occupying enemy territory, basing the bonus on production and distance from cities means that there is little recruitment value in hiding in mountains and jungles, unless you control access to some resource. However, controlling villages and cities is always a good idea, if you can make it work.