I agree with everyone else. If you want Katy to notice you or get some kind of recognition from her, then the acoustic would probably be the best time. Maybe in Unconditionally? Or just before that so that song could hold an even greater meaning. Not sure what the best way to go about it would be, a sign or something, I'm not sure! But I'm sure Katy would say something if she saw, or at least motion towards you or come over. Are you in RS? Or you could do it during Firework, that would be pretty epic too.
Good luck with it. Even if Katy doesn't see or anything I'm sure it will be a purely magical evening for the both of you!! Wish my bf would get his arse into gear and propose to me when we go haha