http://en.wikipedia.org/wiki/Alexander_von_Falkenhausen
This was the head of the German military mission to China during the thirties. In addition to training a portion of Chiang Kai Shek's Chinese Nationalist army along modern German lines, he is said to have taken the field in Chinese uniform to lead troops during fighting against the Japanese, before being obliged to withdraw in order to respect the terms of the Tripartite Pact.
However, this is one, very high ranking, prominent individual. Even as instructors at the small unit level I expect any German advisors to the Chinese would have worn their own uniforms. In any case, reenacting outside your ethnic group is entirely accepted provided you do so with accuracy, so there's nothing stopping you just straight up portraying a Chinese soldier of the period, should you choose.