I use workers in my war effort all the time, though I never thought to abuse them to the point of re-pillaging. Exploit or not, I consider that fair game at Deity. It's not like the AI doesn't cheat (just a little bit...). What I have done, however, is pillage his tiles and then immediately repair them after I take the city. The workers are in the area anyway.
I like forts a lot for those squares 4 tiles away. Your citizens are not going to work the tiles anyway. A tile both 4 away from your city-AND in a meaningful location in a war--generally I find 1 or 2 tiles a game that fit that category. It helps. If you're building up for a war with your neighbor, you should build roads & forts on your border regardless--the 50% terrain bonus supercedes the 1 food/1 hammer. Go back and rebuild the farm/mine later, unbuild the extra roads later. You can send workers to your border all you want without triggering the "Declare War Now" ultimatum.
Workers combine with GG Citadel very well. It goes without saying that you need roads. Then build forts next to it for your ranged units. I just took out the Incas' capital that way, and they were the superpower; hills everywhere, 6 Wonders. Just citadel pop to within two tiles of Cuzco (which pisses him off, but no war), send in workers, build roads and forts. Then send in, park, & rotate your siege. That decided the game.
Sometimes I also chop forest the tiles next to a city I'm sieging, so my ranged units can reach the city. The problem is I might want the terrain bonus for my infantry. You can also chop forest 2 tiles away from the city so you can give your slow units an out in case the bombard turns them red. Classic problem: park your unit next to the city, city bombards you to low health. Forest right behind you, you can only retreat 1 tile, but the city has a range of 2. Bye bye unit.