We've been asked for our ID card number or passport number on occasion for places we wanted to look as well, and I also come from a school of thought where in this day and age of fraud, identity theft, etc, I'm very leary of providing said information.
My husband and I have managed to skirt around the requests by saying we didn't bring passport with us or don't have an ID card yet (which was always the truth in any scenario we were in). If you show geniune interest in places, we found that most people won't push it if you don't provide the information.
If you feel uneasy about giving this info over internet and/or phone, follow your instinct, let them know that you would rather wait until you are in HK and are truely interested in the place before you give them such personal information.
Also, giving a Id number is one thing, but I would be very leary of giving anyone a copy of my passport in any situation...