Evidently, yes. The recently declassified 1946 History of the Signal Security Agency
Volume II The General Cryptanalytic Problems has information on this subject. The section covering solution of Chinese secret communications is pp.180-189. (This is the internal end-of-war report of the US Army's codebreaking organization.)
Messages of both the KMT and the Japanese puppet Wang Jingwei regime were read; the report calls them the Chungking and Nanking governments, respectively. A number of different KMT codes (that the SSA nicknamed CNA, CNB, etc, through CNY) were attacked. On page 185 it says "Two of the largest (CNG and
CNH), used mainly by Dr. T. V. Soong in his correspondence with the
Chinese Mission in Washington (SINODEFENS), had been isolated..."
On pp. 187-188 the extent of results is described:
Progress during the past year has continued in the Far Eastern
field. Complete or partial solutions of four major Chungking systems
were effected, and translations have been submitted in all of them.
Of these four, CNL is one of the principal Chinese Foreign Office systems;
it is a code enciphered by substitution and is the first two-part Chinese code to be solved in the Signal Security Agency.
Of the
other three, CNM (an Air Force code), CNP (a General Staff code), and
CNT (a military and naval attache system) are codes enciphered by digraphic substitution or by keyed columnar transposition. CNT, used
by attaches throughout the world, employs a total of about 400 transposition sequences and at least 18 paginations of the basic code; most
of the traffic is now fully readable as a result of the year's work.
One minor system, CNX, was completely solved during the past year,
and a special system, introduced for the United Nations Conference on
International Organization and made up of transposition encipherments
of several known codes, was partially read.
This work was done with the help of the British "Government Code and Cypher School".
The report does not discuss how many messages were read, nor what use was made of their work.
