Quick update:

Managed to get this linked with Alipay, but only because my friend linked it to my account manually as though it was HK-issued (which makes it process through CUP network rather than direct-to-bank link). He confirmed there isn't any standard, supported way to add US or other foreign-issued UnionPay cards, with the exception of some HK cards. Successful purchase on Taobao.

WeChat is a no-go, as Tencent have never built support for processing directly through CUP network.

10010 and 12306 both say the card number is invalid.

Successfully linked with my CN ICBC accounts, though for whatever reason the US site now appears entirely in Chinese... Appears that I can use China-based USD balances to pay the US UnionPay bill, but not RMB. Still can't find a way to convert RMB->USD balances or vice-versa without showing up in person; CMB has ICBC beat here.