A further question on this topic: can room / suite upgrade certificates (not free night) be combined with the "PMXX" Amex complimentary night + $25 credit rate? I seem to find conflicting information as to whether this is possible when I search.
If it is possible to combine them, what is the best way to book this rate and apply a certificate? Is it possible to book the PMXX rate online then contact the hotel directly to apply the upgrade?
Yes, you can definitely use a room upgrade cert or a suite upgrade cert with a PMXX booking. (I'm not aware of anyone reporting any issues; if I missed a report, could you kindly provide a link? Or is there a chance you're referring to the use of free night certs with the Amex rate? That indeed appears to not work most of the time)
You could either call FPC and make the booking or you could book online. Either way, you can contact FPC or the hotel directly to apply the upgrade cert.