And as was mentioned on another thread, the "requirement" by Beijing for the HK government to spend said reserves on white elephant infrastructure projects so there are not the massive reserves that there used to be, means that the HKMA will not have the means to defend an attack on the HKD. At which point China will support its compatriots from its own 3 odd trillion reserves and buy fealty.
Colonialism by debt is a well established technique by China in developing countries. Looks like they could do it on their doorstep...