Baggage on anywhere else other than the US/Canada is by weight. Period.

So LHR-HKG-LHR is 30kg per passenger. If you're connecting immediately from the US to HKG via LHR and AA lets you check through to HK, then you're safe with however many pieces you check in. But when in HKG returning to the UK, you are to stay with 30kg with maybe 5kg-10kg more depending on the genorosity of the check in counter.

Having OW Emerald is irrelevant. You are not allowed access baggage clearance just because.