I did this once - I put my FF number in when I booked a ticket for my girlfriend just so I could get her a preferred seat - and forgot to take it out. I got a letter from US stating that the flyer and my FF# were not a match and that it was not allowed and not only would I not get any points for it, but if I kept trying to do it, I could be stripped of miles and status. Now this was over a year ago, so who knows what they do now, but I don't think it's worth the risk if they really are serious about threatening miles and status.