American credit cards work just fine with OV Pay for both NS and regional public transport (tried it with a Chase Visa card). Side note: Albert Heijn also finally started accepting them, so credit cards are now accepted at almsot all merchants.
Silly me at the time didn't realize I could go next door to Jumbo and pay with whatever the hell I wanted... live and learn!
[0] https://www.reddit.com/r/geldzaken/comments/1085e6t/albert_h...
Hopefully this will put an end to the common perception in NL that Mastercard/Visa == credit card. Most of them aren't!
Funny note: Aldi accepts everything, even AMEX.