Koan, is your baby a boy or a girl? Reason I ask is that with my son, we never had to bring a portable potty out. I always just make sure that I have a small plastic bottle with me. Convenient, no fuss, can be rinsed out.
With our girl, I haven't come across any other option besides the foldable Tommee Tippee potty that we use (and yes, this one uses a disposable bag).
Were you able to find the Tommee Tippee potty in HK? I checked Bumps2Babes, Eugene Club and Mothercare but could not find one.