Ok, not a bad idea, but I think your not thinking outside the box.
If these generals are on war refuse, and yet feeding their alliance troops, they must be getting their resources from somewhere other than inactive farms. Think about that and try and find a way to cut their supply lines. Soon enough, they come off refuse and then you may get em.
There's always more than one way to skin a cat, if you can't barge through the front door, try and barge through the back door instead.