Few days ago I tried to get home using XJT. The way the loads looked it was going to come down to being able to sit in the 2nd F/A jumpseat. It was
UAX flight and the capt wasn't sure if UA allowed the 2nd F/A jumpseat to be used so he made some calls. The current UAX policy is that no one can sit in the 2nd F/A jumpseat, but they are currently working with United to get the seat available and hope to get it done soon.
COEX=any off line pilot can sit in the aft FA JS
UAX=nobody can sit in the aft FA JS
If you are trying it on COEX and the CA doesnt know you can(i.e. isnt a commuter), its in the FOM now if I am not mistaken(dont have mine at home with me).