Proper highchairs? Like snow in the sahara! Here are a few I've found:
Central: the Landmark Three-Sixty (can you believe that's the only place I've found in Central???)
Admiralty: Dan Ryan's at Pacific Place
Wan Chai: Fat Angelo's, Starbucks at the Hopewell Centre
Causeway Bay: Tony Roma's in Times Square
Peak: Cafe Deco, (and if memory serves, I think the Starbucks there might have some too - not all Starbucks do)
Repulse Bay: the Verandah
Stanley: Pacific Coffee
Please, would be great if everyone could add to the list. But as you say, a safe highchair with a 3-point or 5-point harness is extremely difficult to find. Really bugs me!

They only cost about $200 in Ikea.
Hope that helps a little