I can tell you from experience that at least some seats are available for PMUs on the ATL-NRT-ATL flights. I have made that run before and usually have the luxury and flexibility to choose my travel dates. I agree with heckler that this flight would be simply unbearable in coach, so I phoned SMS, gave them a window to work with, and they found seats on the outbound and return. I booked on the spot and had the BE seats I wanted. I can't speak for every flight, but PMU seats in BE have always been obtainable for me within a few days of my preferred travel dates.