Asia News and ZENIT report that Joseph Card. Zen, Bishop of Hong Kong and all around excellent fellow, says that the Pope’s letter to Chinese Catholics should be ready "around Easter". Asia News has a somewhat more complete picture, saying that the letter could be around 20 pages in length.
According to Asia News Card. Zen said "… a prudent dialogue is possible with the Chinese government but “on religious freedom, the Vatican should have a clear stance. This is not an irrational request.”
With reference to the way bishops are appointed in Vietnam Card. Zen suggested that the Vietnamese model could be a good reference point for dealings with Beijing. Apparently the Holy See presents the names of possible bishops to the Vietnamese government, the government responds with feedback, and the Pope makes the final decision. There is nothing like the government approved Chinese "patriotic Church" in Vietnam.