I have a simular system made by Held (see:
http://ducatimonsterforum.org/index.php?topic=9946.0)
They also have a setup where you attach the GPS to the bag. I've never used it (don't even own a GPS) but I don't think it's very solid though. The weight of the GPS in my opinion is just to much to be held stable by the bag itself. I wouldn't trust it and would imagine the GPS bounces around during riding with that setup.