The xONEx rules still say
5. RESERVATIONS AND TICKETING - (a) Reservations and Ticketing
Reservations for the first international flight and all preceding flights must be made prior to departure.
Subsequent segments may be open-dated.
I've never tried this (on about 7-8 RTWs) and I'm wondering if it's as easy as it looks. Do the airlines (I'm particularly curious about the AA RTW desk) accept such bookings without argument, or is there pushback (or flat-out refusal)? We know that some carriers add additional hurdles that are not in the xONEx rules and, indeed, may even be counter to the spirit if not the letter of those rules -- e.g., own-metal requirements. So, I wouldn't be surprised to hear something like 'Nah, that's a legacy from the days of paper tix. Nowadays on the computer everything has to be specified from the beginning.'
Insights appreciated.