Obviously, if they don't see you, they will leave you alone. Same if they see you but don't notice you doing anything unusual.
I thought that photography was theoretically forbidden in HK MTR stations and trains but I just Googled the MTR bylaws and skimmed them and didn't see anything: http://www.mtr.com.hk/eng/legal/images/mtr_by_law.pdf
Update: Just looked again and found it under 28H(e) that forbids filming without MTR Corp. approval.
If you really want to try, I think that you'll have to avoid not only MTR staff but also the gaze of the in-station CCTV cameras. The hawkers who infest the stations do this by setting up inside alcoves, but that probably won't help you since you're trying to capture a mass crowd of people rushing between trains.