hi there
research of Hong Kong can be successfully achieved on many levels...... As a former British colony a lot of the later births deaths and marriages appear on a microfische found at larger genealogical societies and large libraries throughout the world..... if you live in the united kingdom go to your local registry office and make enquiry where they have copies of the indexes stored. Next check the indexes of findmypast as some civil registry appears on that website.
I strongly suggest a post on the HK boards of ancestry.com as they have a high return of answers.
A lot of Brits in HK were actually in the services with their families and i would ensure that the people you seek werent actually born in UK or India for that matter.
you say you are after material from about the time of the Opium Wars....... there are some probate references on anc if the probates were settled in London or the Prero Court of canterbury.
Also check the war records.......... of the NAA site
I would strongly suggest the writing of letters to the HK library once you have something to ask....
A lot of folk that went to HK in the period you seek were also of the East India Company and I suggest checking their lists....
https://familysearch.org/learn/wiki/en/Hong_Kong this link offers near little but check with cindis list........ also type directly into google with surnames of folk + HK genealogical databases.....
i also note that you suggest that the authorities had zip....... well civil registration was not manditory then and you need to contact the diocese churches in HK....... for christenings etc...if in fact the family used them at all
good luck
fastfusion