I suspect that if there are active troops in the province a simple way to deal with foraging would be to permit the grounds troops to get a few rounds of combat against any raiders to represent various small scale conflicts, or to simply ban foraging anywhere where there are active castles without very large numbers of naval units. Of course foraging ones own provinces would be somewhat different, if odd. I'm not sure how common this would be as an action - generally foraging in someone else's land is begging for war, so I'd only expect the Vulkan islands, or perhaps Rjuvik to do it, in which case it could perhaps be done as a contested action.