You could always try using a OW alliance member to see if you can change or select your seats with them ahead of time. I've always used either Royal Jordanian or Qantas for this. Get your record locator number, then go to the website for RJ or QF, select manage my booking, then go down to seat selection and see if you're able to choose. The only time this didn't work for me was on a C trip I booked on LAN using Avios. For award tickets in J/F it has always worked for me though, including on CX (though I realize this isn't a premium cabin situation, I think it still should work).