So here's another wrinkle to this. I have an Avios coach award ticket on US Airways for an int'l flight leaving the USA. I switched my ff number to Qantas, where I am a sapphire elite. It got me better seats. I went to check in, and entered my passport info and such. But, unlike a domestic flight, it didn't offer me the option to add my Known Traveler number. Concerned that I wouldn't get Precheck, I went to Finnair and switched my ff number to AA (no elite, but I get zone 2 boarding by having their credit card). I still didn't get a chance to enter my Known Traveler Number, but it's in my AA profile.
I got precheck from doing this. Had I kept my Qantas number, is there a good chance I would have been denied? Or is this just silly?