I'm Platinum and I'd doubt the likelihood of getting a complimentary FC upgrade after standing by, although I think the gate agent can do it and just about anything. I echo the advice to get a good seat assignment on your booked flight. You could invest the time to go early, stand by and hope for the first flight to clear coach and then the FC complimentary upgrade. If that fails you could elect not to take the standby and just be early in the silver line for a no show on your booked flight. I don't find many domestic flights that are not full in FC.