Mike, Good for you for wanting to share gratitude for a government employee's efforts. Public agency officials rarely receive traditional for profit business style perks (raise, promotion etc) for a job well done and as you know, "gifts" from vendors are not allowed. A thank you goes a long way. I ran a business which served the public sector for 11 years and was always amazed by how a simple thank you was met with much appreciation. We used to create "certificates" for our client contacts. I was often (pleasantly) surprised to see the framed certificates hanging on the walls of our clients' offices! A pat on the back felt really great to these folks and in turn helped us forge some great relationships. Good luck!
A well written formal thank you letter specifically showing how the person went above and beyond the call with the immediate supervisor cc'd sounds solid. I would also contact the head of the agency and tell the story because they seldom hear and always appreciate good news of this kind.