During online checkin the whole seat map becomes available - but only with whatever is left. I think once the system goes to load control 48 hours prior to the flight, certainly you cannot adjust seat assignments on line from that point.

I suspect that once load control are running things, SSRs (special service request) and other - ahem- ‘special’ seating requests are then processed. Online check-Ian possible 24 hours prior.

I know when Mrs TK travelled AKL-BKK last month on a W fare she could only select the rear half of the main cabin during booking. Hiwever online checkin allowed her to access the remaining seats anywhere in the economy cabin - she could select 34D from memory.