There have been earlier threads on this topic:
Car Seats
and for older babies:
Child's (Taxi) Seat Belt Vest
Basically the reason why in many Western countries special car seats are mandatory for babies is that they are much safer than anything else. The only other safe alternative approved in the UK is a carry-cot that can be strapped onto the back seat (not sure if this is available in HK). If you don't want to lug a car seat or carry-cot around, you are compromising on safety. Other solutions won't help you much in a head-on collision, and things like the Baby Bjorn carrier could actually make things worse in such a situation, because the baby's head is just below your chin and your head could slam into the baby's.
By strapping the baby to your body, the main risk you are cutting down is the danger of your baby slipping out of your arms in a sudden braking situation, which is obviously much more common than a full-on collision. The most important thing to remember is never put the seatbelt around the baby. Many people's instincts would be to do this, but all you would be doing is crushing the baby with your bodyweight against the seatbelt. Put the seatbelt around your body, underneath/behind the baby. (You may need to weave it between the carrier's straps to do this.) And I'm no expert on safety but I reckon that slings (where the baby's body is held somewhat horizontally) may be safer than upright carriers because the baby's head will be further away from your chin. You can buy Maya Wrap slings through La Leche League in Hong Kong:
http://www.lalecheleague.org/HongKong.html
BTW most people leave hospitals with their babies loose in their arms here, and I was advised to do this by nurses! Safety awareness here is quite low. I think the safest alternative to a car seat might be a carry-cot (which you can also use as a bassinet at home, and some stroller systems come with a carrycot for newborns), stowed on the floor of the taxi if it's not designed to be strapped into seatbelts, because if there is an impact or sudden braking then the baby would most likely be thrown upwards towards the padded back of the seat in front, better than some other scenarios.
Sorry if this message is a bit gruesome. Most people relax their safety ideals a little in Hong Kong because the alternatives are too impractical, and the chances of a serious accident are so slim. The fact that traffic speeds are normally fairly low must reduce the risk of serious accidents, and we Islands residents spend so few hours per week in cars/taxis that we would have to be incredibly unlucky for something bad to happen to us.