I have made same day CW and F miles bookings (using the method mentioned in my earlier post) many times and there has usually been availability when I have wanted it.

Much more curious was a same day miles booking made on a flight which turned out to be oversold. I booked a same day CW miles seat on a route on which there is only one flight per day. At the airport it became apparent that it was oversold in CW and my very courteous request to the local station manager for an op-up to F was met with a smile and a new BP handed to me in the lounge, on account of my Gold status . Now go figure that for yield management !