Sven, thanks for posting the Mazda image. I'm surprised to see a current car design being tuft tested!
Also, i had never seen that 'air curtain' method of seeing the tufts behind the car.
I imagined a way that the flow could be seen behind and above the vehicle : add a tall 'spine' down the center of the car and thread your tufts through the spine.
BTW, I also have seen a method of tufting the area at the hub area of a wheel : Apply tufts to a strip of tape and pull it tight from one side of the wheel well to the other just above the hub of the wheel. Just don't plan on doing this with the front wheels unless you are not going to turn the wheels !
Now that i think about it, an even better way would be to tie your tufts to a piece of thread and then attach the thread to either side of the wheel well.
As far as interference from other tufts, i know nothing of aerodynamics so that was just a guess, but it cant hurt to stagger them 'just in case" !
If anything, it will keep the tufts from snagging each other.