It's actually pretty easy. As a frequent flyer with status on an airline based outside the US, you have access under the same terms as you do at home. Therefore, if you're a BA frequent flyer at Silver or Gold level, you have access to all the business class lounges on any itinerary within North America.

Gold additionally have access to the AA Flagship Lounges and any First Class lounges that may be in North America, again on any itinerary, no restrictions.