OK, mocked up something so as not to damage the real stone guards.
First hole nearest the edge of the wing is perfect, so trick is how to secure with only one pin. I've not removed the first pint yet or anything, but on the face of it, the arch liner seems to be secured by the pins pretty tightly.
The downside is that the wing lip on my car is very thin, no where near as much to play with as on the photo kindly supplied by Scooby_doo.
However, I wonder would the tightness be enough to rely on?
John N, the distance between the pins on my E63 look to be about 62mm also.
Another thought, I wonder, whats to stop me installing the stone guard in front of the liner, then I could actually put some kind of adhesive on the back of it and attach to the liner that way, together with the first pin to secure it, the the double sided tape to attach to the arch lip?