I am with Tango's 1041pm post ... don't give a ff number but do so on the last segment if you have the time. Why complicate matters? Can you imagine if you are late checking in for a flight and didn't have time to change the ff number on file? I'm saving my boarding passes and sending them in after (by registered mail!). Simple and headache free!
P.S. Since I have no status with any oneworld carrier I'm not concerned about having my ff# on file to be recognized for upgrades. Then again all my qualifying segments will be short haul flights of less than 2 hours so it shouldn't matter! (My transatlantic flight is an Air Canada Websaver fare with an already confirmed upgrade to Executive First. Yes, I'm a *A crossover!).
BB ... sorry you aren't able to to find the time to join us ... would be nice to have another *A crossover! Will send you an e-mail from HEL & BCN!