Thank you everyone for the insight and perspectives. I have not done this in HK at a Marriott property but may play it safe this time around.

I phoned both Marriott and the hotel in question (Renaissance). Firstly, the e-cert cannot be transferred. In addition, the first US-based Marriott agent said adding a name could not be done. However, I phoned back in and the second agent not only added a second name to the remarks, but even requested the separate reservations to be joined so as to keep the same room. I phoned the hotel and the local reservation agent confirmed the secondary name in the remarks would not be an issue for check in but confirmed explicitly with me that the e-cert holder is indeed staying. This strategy may work for one night but may be more difficult for consecutive nights and unlikely worth the hassle. However, Mobile Check-In and Key functions could facilitate this if the hotel is equipped... If anyone has tried this, would appreciate the feedback for future.