Otherwise I don't know if you'll find an open source alternative to Google wallet, since the banking system overall is very much the opposite of open source
They have to be certified by Mastercard/Visa, so no chance for open source. A few banks have their own implementation. Apart from mentioned samsung has its own version.
that's fine, but really. Why not just throw any 3rd party out? It might be more confortable to grab ur phone than to grab a credit card, but that's as close as you can get to perfect (with zero effort even).
Do not know of any open source alternatives and it is surprising to me there are none. To my understanding, all that is needed as a mobile app that one can store their credit card on (number, expiry, cvc) and have NFC for permission for tap payments. The app should be able to function offline since the payment is done though the NFC, not your phone's internet connection.
It's a bit more complex than that because for contactless payments you don't share the card information with the vendor but an authorization token that need to be verifiable and certifiable by visa or mastercard.
So it needs to be sponsored in a way by visa or mastercard.