FOCAL,
Sorry, not trying to be antagonistic. The truth is that a "foreign" registered aircraft outside of U.S. airspace doesn't have to obey FARs even if it is operating to or from the U.S. Thus an aircraft operating from, say, Germany to the U.S. will have to have the FAA mandated equipment on board (else it couldn't be dispatched from the U.S.), but will not have to follow FARs regarding diversion until or unless it is in U.S. airspace - it follows the LBAs regulations (to complicate matters they may not be the same as the EASAs regulations... there may be requirements on top).
Thus regardless of whether or not the aircraft is a twin or a quad (or a tri), it has a D registration and will divert according to German rules, not FAA ones. That means that for a quad, it will divert where it likes according to the situation. For a twin, it will be the nearest alternate. By the time it gets to U.S. airspace, it won't be in the ETOIPS portion of flight anyway, so the whole thing becomes academic.