I dont think Kurt had one his his JS, but had them almost on all the sonic blue mustangs he had i think. They do look cool though on a JS, I was looking for one myself but apparently the sizing is funny. I have had a look on warmoth but I dont think their pickguards are the correct size or screw positions as the Fender Jagstangs, they are made for their Warmoth jagstangs, but again I may be wrong. The one on ebay looks ok, but the screw positions might not line up correctly so may require a bit of drilling.
If anybody does know where you can get JS pickguards that do fit directly to a "Fender" JS i too would like to know.