I have one with a mystery limp, too. Clean xrays and I'm still worried to death. Fortunately, while my Vet is just a little stubborn when it comes to buying into the "greyhounds are different" credo, he will do EXACTLY as I ask and will search to the ends of the earth for answers. (I secretly think that he searches just as much to prove me wrong as anything else. ) So, if you think you have a Vet that's receptive, I would use the resources of GT and use search engines to see what other information is out in cyberspace, print it out, take it to your Vet and be specific with your requests/demands.
In the meantime, we're sending prayers that after Friday, you will still be dealing with a mystery limp.