From what I know, NAB's Foreign Currency account is still single currency. You need 10 accounts if you need 10 currencies, with $40/month fee if balance is lower than $20k. The benefit is with their Merchant Account, which can offer settlement to foreign currency accounts with a single interface.

I've never seen any other major bank in Australia publishing any foreign currency offering for credit card processing.

Braintree is not a small player, so it's highly likely that they are able to get customized proposals from different banks and the choice of NAB should be a lot more than just the existence of a published product.