I know it's possible but it's going to be a real PITA, becasue you have to work around the hub and......it just won't be pretty.

 

If you have an excellent grasp of what you are doing then you can do it on the car, no sweat. Otherwise, its better to just take the arms off to change the joints.
With the regular snap rings installed, you can have a much easier time doing a set on the car. When working with the "no holes" porsche oem snaprings, I would suggest just pulling the arms.