I just (unintentionally) found that you can at least do a partial payment on any credit card.

It happened because the regular booking process is broken if you want to buy a roundtrip ticket in N class (in order to apply GGUs). The first flight will revert back to lowest fare in the checkout page Probably something they did not test with the new 'price is now roundtrip' logic.

So basically, book (any?) roundtrip with the companion fare, use the BoA AS card to pay, then make a change, and that page will accept any credit card for the additional cost