Tough. If you can't confirm F on any sector at ticketing, you have to waitlist for F. It then has to clear prior to departure, and the ticket has to be reissued at the F price. Not pretty.

The preferable option is to be able to confirm *something* in F. You can then waitlist any unconfirmable segments, and, even if no XF opens up, you can go on the DM list at the airport.