Wow, thanks for all the valuable info! GibSpmuh, rebooking PER-SIN-LHR on BA is an excellent suggestion which makes the Platinum Challenge very much worthwhile. I think Dave Noble is correct that it would only be 25% of miles flown, but still better than nothing. He would then reach platinum with the JFK-SEA flight, and enjoy the bonus points and status privileges after that.

But without rebooking those QF flights he wouldn't make it to platinum at all (nor would he make it to the 20,000 miles required to redeem a reward). I'll let him know about the challenge, and encourage him to investigate rebooking on AA/BA flight numbers. If it's possible for him to do so, then AA is definitely the way to go.

Thanks everyone!

Steve