I think the ID you are referring to are not relative to visitors. I only carry my drivers licence with me plus some stationery from the hotel I'm staying at. I've never been requested to show ID and I have walked past Police on the way to 141 walk ups.
Why would you carry a lot of cash. I usually stow the majority of my cash [including credit cards] in the safe in my room and go mongering with only the money I figure I'm going to spend. [also helps me budgeting as icould spend it all at once otherwise!]
When are you visiting HK?