BA's [remove name of staff member] confirms that BA Boston honors the Oneworld rules for access. He tells me: "If you are a OneWorld emerald or sapphire traveling on a OneWorld carrier you are entitled to our lounge in Boston." He didn't mention class-of-service based entrance, but if elite entrance is fine, surely class of service entrance is also permitted.

Wish I had been there in person to see the denial and hear the agent's stated rationale (ideally, transcribing it verbatim). I will try to take IB ex-BOS sometime soon, both for their reasonable business class fares (usually easy I inventory) and excellent new seats, and to check on this issue. If this turns out to be a one-time fluke, I will be much less likely to pursue it.