Those who are against the wall just go around and build the wall north of their property. If they want back in, they can find the big beautiful secure door.
There are currently many sections where a fence exists and a portion of an American’s property is on the other side of it. They have to get permission and arrange for a BP agent to open a gate while they come and go.
So it isn’t really a strange notion to do as you say and build the wall through whichever property north of them is willing to sell first. It won’t change where the actual border is, just make their lives difficult coming and going through the wall.