When I was doing my RTW quite a few of my BA segments were waitlisted at the time of booking. What I did was to confirm an alternate routing, then waitlisted what I wanted. I then called every few days to see if it'd cleared. On one segment I asked CX to call BA to get it cleared as I had no other alternative and the flight was wide open in A class. It took two weeks but was done. IMHO unless the flight is booking up you have a good chance of clearing a waitlist if you work at it.