I don't like OP's odds. While he's arriving as a domestic pax at IAD, UA requires intl. departures to be at the gate no later than T-30. That's 18 minutes from brakes locked on arrival to the NRT gate. The MCT for IAD for UA on DOM-INTL is 35 mins. But, Intl-Intl. is 50 mins.

On the grounds that this is an intl-intl connection (even though it's preclearance), OP has an argument that this is an illegal connection and that the 3rd party website needs to rebook him without fee. Given that there is hardly hourly service, this isn't about just hopping the next aircraft.