Today, while purchasing a US Costco membership, I noticed that Costco online shopping has started accepting UnionPay cards. Previously, it seemed to support only Visa/Master/Discover. Now JCB, Diner's Club, and UnionPay credit cards have been added. It's unclear when this started, but see the screenshot below:
I tested it with a Bank of China mainland China UnionPay single-network credit card, and it went through without any issue. Interestingly, after the purchase, the interface displayed the credit card as Discover.
Now that US Costco supports UnionPay credit cards, you can use a Chinese UnionPay credit card at Costco for overseas online spending and potentially earn elevated cashback rewards. For example, you could buy a Costco Cash Card online and then use it in-store, effectively converting it.
PS: Mainland China UnionPay cards do not really have a billing concept, so you can just use a US billing address. The only thing to watch out for is that your bank in China may call to confirm that it was really you who made the charge.