Apparently there is a change to how one can pay for the CoPay in 2017. According to a recent booking, the usual $600 copay for an international leg can now -as of 2017- be paid in the currency of the credit card's billing address.
This particular reservation was FareLocked with a fare in RMB and what is referred to as "Upgrade Award service charges" also in RMB.
HOWEVER, when I wanted to pay for this reservation, the Verified By Visa page requested a charge of about 5 times the supposed charge.

A call to United and 30 minutes later, the CSR was able to issue the ticket by first charging for the economy fare, then, once ticketed, upgrade charging the "Upgrade Award service charge". Oddly enough this was in USD though, not RMB.

So, it appears that there is an official United policy of allowing the CoPay to be paid in the CC billing address' currency, but it does not work properly (at least for me, and having FareLocked the reservation).

For what it's worth.