NRT-SIN-NRT on AA codeshare with AONE5
 

Is it possible to book NRT-SIN-NRT on the AA coded JL flights? I'm routing


HKG-NRT (CX)
NRT-SIN (JL/AA?)
SIN-NRT (JL/AA?)
NRT-HKG (JL)

HKG-JFK (CX)

the ticket is an AA issued AONE5. Subject to availability, can they force the AA codeshare?

Yep you can but dont' know whether you can get A availability that quickly. Also I don't think NRT-SIN have First class?

IMHO, No.
NRT-SIN AA code share only avaialble for connecting to/from transpacific AA flights.
ExpertFlyer only shows JL flights for NRT-SIN, but shows AA code share for DFW-NRT-SIN

I don't think you can.
I'm currently working on a DONE4 and wanted to book the AA codeshare on BA from MCT (Muscat) - LHR, but the ATW agent said you can only book an AA codeshare if you are connecting to/from an AA flight.

Happy Travels.

Not with your routing. As stated those codeshares are only available as part of a transpac flight on AA.