If you find someone who can use the native res functionality, LNER trains show up under airline code 9B (for Access Rail), it appears as a pseudo flight. There’s no reason agents in the US can’t rebook onto these, other than that they may not be aware of what it is. If you tell them you do and are happy with the option, they should be able to do it, but equally it might just be easier to sort on arrival.

Theoretically tickets are only valid to the city described but it’s hard to see how you wouldn’t get away with getting off at York.