As suggested, I would call SAS directly and try to work through them to book your seats. However, don't be surprised if they cannot book seats on certain airlines. I had that happen to me on a few airlines on a RTW ticket. I wasn't able to book a seat on SQ out of FRA to SIN (but got a seat on SQ out of JFK to FRA) and I wasn't able to book my seats on NZ from SYD to AKL and LAX and nothing on any of my U.S. sectors (on US Airways). So it was a bit of hit and miss (on a paid C ticket) but at least I got window seats (my preferred seats) on all but the AKL to LAX sector on NZ. Good luck and enjoy your RTW trip...