Depends on the airline, but I've never been asked at immigration in PEK or HKG. For example, I've flown one way into PEK on AF and was never questioned. Yet, I've flown one way from SYD-HKG on VS and I've needed to show proof of onward travel. In one case I didn't have proof (b/c I was picking up a ticket in HKG), and they gave me a lecture and let me through.

I think you'll be able to weasel your way out of the airline by saying you will purchase the ticket later, but you may need to sign a waiver.