Pharmacies are very prolific in HK. I'm not sure how they'd react to a prescription from elsewhere, but there's only one way to find out. Also, if the first one rejects you, it never hurts to try a second.
Assuming pharmacies do reject you, I would bet that at least some pharmacies could refer you to a doctor. My one medical experience in HK was much more like the US than China. I had to go see a doctor (who had a fully separate office) to get a referral in order to get checked in to a hospital. A dear friend made the arrangements, so I still don't know how to find a doc in HK all by myself.