Neat solution, indeed.
Geeksim created a "water barrier" with hanging chains which seem to work as well.
Hanging ropes do hold back animals too, so a nice, impassable "curtain" can be created (visitors don't mind it!). (Idea by Mike Sheets)
One thing though:
I am wondering, if you won't get false escaping alarms, as your "Null fence" is in front of the impassable terrain (the rocks). When I look at your second post, there do seem to be little places, where animals might go that are behind the virtual fence.
It really doesn't matter, where to put he fence. Personally, I would move it a little bit back, just to be on the save side.