A quick update: Just off the phone with the Gold Desk. The guarantee only kicks in if the flight is sold out in all classes of service. So it is not sufficient if economy class is booked up, but business class and first class need to sell out as well before I have a guarantee with my flex eco tix. The Gold Desk can then make a seat available for me within seconds, they said, provided I am still outside the 48h window.

Unfortunately, there are still 2 FC seats for sale on the flight that I plan to take. So I now need to hope that they sell soon....