Freds, you're right for a regular fence, I was thinking about one of those invisible fences that are buried. Most chain link fence is buried 6" or so in the ground, I suppose that a middle buster or a one bottom turning plow would work OK if you did it before you put in post.
When I was small, my Father had a hog that he said would run under an electric fence after the hog worked up his courage. Aparently the hog knew that he would be shocked, but would run under the fence anyway. My Father put a wire collar on the hog with a turned up hook, he said that the hog learned very quickly after that and didn't go near the fence again.
If anyone is interested, The Progressive Farmer is a pretty good source of information for hobby farms as well as full time professional farmers (not many left any more). They have a section on reader ideas and inventions that has some good ideas, just good old fashion thinking.