From talking to another mum who went the private-room-in-public hospital route - in QMH - even with the backdating it works out to around hkd50,000 for a natural delivery, which is way cheaper than a private room in a private hospital without antenatal charges counted. QMH seems to have the best deal though - because you can attend private antenatal appointments in a private clinic, have a good chance of delivering with that doctor, can reserve the private room in advance, I have a feeling chances of epidural are better, and then the advantages of the private room are you can have your husband and baby with you.
It's an option I would have seriously explored if I hadn't woken up to it so late. Now, in my third trimester, I've decided to just go with what I already decided.