Hi Max,
My FOM simply says an airport must be "Available" and I know the common interpretation of that is "Adequate" (No weather requirements). Do you have a reference that is different? I'd be very interested to know and would have a serious look at our regs.
One thing though, I think people get caught up and think that "Adequate Airport" is a term soley associated with ETOPS planning. It isn't. It PRE-DATES the invention of ETOPS, and the fact that many routes couldn't meet the "Adequate with 60mins SE flight time" rule was why ETOPS was invented in the first place.