One additional quick note, based on what you wrote: "I know one of the options is to input a Line barrier like the 49th parallel across or a poly barrier, but it would be simpler to just filter on an attribute already in the HERE dataset"
If you have edit access to the network dataset, then you could add a restriction attribute and use that to stop traversal into the US. But, if you cannot edit the data itself, then you will have to use a line restriction, or some similar solution that affects the analysis layer and not the network itself.
The definition query, if you are discussing what I think you are, will only affect where the facilities locate along the network. Once the facilities have been located, there is no place to add a query that will restrict travel along the streets.