A friend lost his license and went to the traffic department to get a duplicate issued. They charge initially 10 or 15 riyals to issue a letter which can be used temporarily for 2 or 3 weeks. At the expiry of that letter, he took it to the same place and they issued him a new license, he had to pay 250 riyals. He did not have to supply them with any letter. Maybe his case was different, but they did not ask for any proof or ask where the license was lost, etc. So as a thought, maybe you can try the same route ? :S