It depends
On 400s to Asia, some are double augmented, meaning that there isn't a jumpseat to OMC on. On the ones that you can OMC on, I believe you need to call operations and list yourself 24 hours in advance so that you will be put on the General Dec.
I don't have my Flight Ops Manual with me to check for sure, but I believe that's the way it's being done (as of today). Who knows about tomorrow.