At least as far as I have observed, It's very rare that a non-Z C pax can't get upgraded into GF. Maybe not in advance, but certainly at check-in or the gate. As long as the probability is almost certain, who cares whether it happens at the gate or in advance.
Also, while the UG list for F often appears quite long, it is mostly non-revs who are prioritized below anyone trying to use miles or an instrument.