This should be quick and simple~
Flipping a camp that feeds into a keep or killing a yak headed there should qualify for at least bronze level contribution.
If I can go a little deeper, assault events can have sub objectives that include owning the surrounding camps and killing spawned yaks. This will more effectively communicate to players the importance and value of cutting off supply. Perhaps the reward for flipping the keep can scale with how many of the support objectives have been completed.
There’s not much potential for abuse—anyone who gets credit in this way could have just blobbed into the lord room for the same effect.