Going into Timatic shows that for Taiwan, "Visitors must hold return/onward tickets", so that's it. I suppose for places where this is not a requirement, OLCI on Cathay wouldn't be a problem. Yes letting passengers do OLCI and not giving them access to the boarding pass so that the ground staff can check documents first would be much better.

Another big glitch on Cathay is that for places where you need a visa and the visa happens to be electronic and you just print out the visa, you cannot "scan" the visa into the check-in kiosks at HKIA. Must see an agent to verify the docs before you get your boarding pass and having bag tags printed.