When I have booked my DONE4s through CX, I have had to specifically request certains seats as none have been pre-allocated. I did this after ticketting. Simple enough task. The emailed itinerary that CX has returned to me has shown the CX seats. They told me that they cannot book other OW seats. All I did in that case was to ask them to quote me the QF PNR that was generated, I rang QF and booked QF, BA and IB seats. I also rang AA in SYD to request the same.

The latest CX BKK would allow ticketting after making reservations was a week before travel - for reservations orignally made 3 months prior. I suspect that may be a bit negotiable, but they absolutely certainly would not do it the day before.