Countries buying Chinese products get Chinese products by paying for them.  If they pay with HK$, that means HK$ is the currency used in China.  A currency can be a global currency when it is used by a lot of countries for transactions.  The US was able to maintain its currency hegemony mainly because of the agreement with Saudi Arabia and subsequently OPEC to transact oil purchases in US$.  

Hong Kong with a population of 7+ million people, no longer a gateway to China, has little industries and transactions between HK and other countries are relatively miniscule.  So, in what way countries can use HK$ as a global currency?  It will only work if China also uses HK$ instead of Yuan which negates your original plan of having your cake and eat it too.