FWIW I obtained a one year, multiple-entry visa (30 days per stay) with the the following from the visa office today in Wanchai:
US Passport
Non-permanent HKID
22 months left on HK visa
6-month China multiple entry (expired) in the passport.
My kids did not have a previous multiple-entry China visa in their passports and were only given 6 month multiple entries. After they expire I was told them can get a one year multiple entry.