I don't know of a working address, I have been trying him for ages but i came across this while searching. It explains a lot about why they are no reported success and proberly the only way you will be able to get his autograph is in person which is difficult when he doesnt like to promote his movies:

http://images.google.co.uk/imgres?imgurl=http://www.christopherlloyd.net/christopherlloyd/bttfdoc_sm.jpg&imgrefurl=http://www.christopherlloyd.net/christopherlloyd/stats.html&h=207&w=194&sz=9&hl=en&start=2&um=1&tbnid=K-nmUaOcgYXsoM:&tbnh=105&tbnw=98&prev=/images%3Fq%3Dchristopher%2Blloyd%2Bautograph%26svnum%3D10%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26channel%3Ds%26rls%3Dorg.mozilla:en-GB:official%26sa%3DX

I notice there isn't any success stories on his page, does anyone know of any success, perferably recent ones? Thanks!