I don't see how having a HK qualification is going to make any difference to what type of visa you get. You can only apply for an employment visa if you have a specific job offer - the employer must be your sponsor for the Employment Visa application.
I notice also that HK MLT practice certificates are only good for one year and expire unless renewed (
The Medical Laboratory Technologists Board)
I know that HK is desperately short of nurses, but I'm not sure about lab staff. However, I do see that there are quite a lot of MLT vacancies on some recruitment sites, e.g.
MLT jobs in Hong Kong - recruit.net
I don't know whether the Immigration Department would grant an Employment Visa for such jobs, but I guess the only way to find out is to try.