Dont really know but this is my experience?
New san diego: It was quite late, about 1.30am? Just walked through reception and no questions?
Old san diego: Got asked who were meeting and needed ID to register?
Regal Kowloon East: Quite late too but walked straight in and didnt go through the main lobby but the side entrance?
Empire TST: Doorman opens door for you and i just walked straight to the elevator passing reception?. No questions asked?
Not sure if things have changed recently and someone posted about OSD & NSD in the chinese section about having to register with the hotel even with 1hr punts and CID accompaning to which room you are going to? Scary!!!
Probably best bet and get your own room and have the girl come to u?. Maybe let reception know that your going to have a guest staying?.