Not nearly as exciting as JFK-YVR-JFK CX F, but I did just book:
GRX-BCN VY (IB) Y x 4 seats. (5k Avios + $22.21)
Thing is, I still need a 5th seat. EF says Y4, so that must be the 4 seats I just booked.
What's the chance a 5th award seat will open up, and if it so improbable, when should I buy a revenue seat? (Current price: $112)