Let me check with my dog people and see if there is something over there.
You can also check out
http://www.***************************. The Admin has a yorkie that does "Go to Ground"(sane as earthdog) with American Working Terrier Association.
BTW, don't know if you know it but yorkies aren't allowed to test in earthdog with AKC. There has been a big debate about it but it is still a no go situation.