There's no way they'd let you place a citadel in an enemy's territory, let alone flip 7 of their tiles from anywhere in their empire.
You can't do that, because the tile must be adjacent to your own territory. The question is whether the tile adjacent to your own territory must be neutral, like with the vanilla GA culture bomb, or that that tile can be someone else's territory, probably someone you are at war with. It is a Great General after all.