This thread has been added to an older one that says that waiting is the wrong thing to do.

You should call Amex and ask them if they can apply FHR to your existing booking. This needs to be done before you travel, at the checkin desk is too late, as Amex needs to do some work before you arrive.

I'd love to know how this turns out. It would be great to get a good SPG rate AND the FHR benefits too!