I've seen in various BMW parts catalogs, that on some bikes there is a ring type metal 'gasket' that is installed between the rotor and wheel rim . I don't know how thick the part is, but it may be just enough to do the job. I don't see why you couldn't add a shim between the fork leg and caliper, I would make it large enough to cover the entire contact area between the fork leg and the caliper, not just the size of a washer.