My strategy was that I desparately wanted to be a pilot, but failing that, I'd settle for engineer just to get me in. That covered the 'officer first, pilot second' angle whilst enabling me to show no interest whatsoever in being an observer.
Like yourself, I've done a fair bit of flying already, so if I'd been given a hard time about not wanting to be an observer, then I could have spouted off about my love of hands-on flying ad-nauseum. However; in spite of the fact that I had equal P and O scores and the fact that I had on observer on my board, the question never came up. Needless to say, I never ticked the observer box on the application.
After AIB, they give you a form on which you can say which branches you would or would not accept. From then on, you can afford to be a bit more blunt. I also called the selection officer when I got my confirmation letter. He suggested that I might be offered observer because of my age (25) and my ropey AIB score. Again, I just said I'd hold out for pilot. One week later, that's what I was offered.