After giving closer observation of the letter from Shenhua's mysterious stepfather (through it's texture), which is found at the end of Shenmue II, it became clear that a signature is actually provided at the end of the letter. Apparently, the name of Shenhua's stepfather is Zhongshou Yuan (given name first). Since his name is never mentioned, nor is the player able to obtain a well enough view of the letter in the game, this information comes completely new to us. However, keep in mind that we cannot positively confirm whether this is to be the final name or not.
Unfortunately, the name doesn't give us any specific leads to anything like character relations, but it's still important information. If I had to guess, I'd say that his name will be pointed out soon after the beginning of Shenmue III, as Zhongshou will definitely be brought up after the current events. And as we know, a fairly large amount past Shenmue II has already been designed, so it's not really a surprise that his name can be found on the letter in Shenmue II. But remember, like I said, we can't be 100% positive that Zhongshou Yuan will end up being the final name, though the chances are rather high.
A special thanks goes to Kiyuu for the translation.