Don't quote me, as I've never seen a concert there, but it sounds like GA lines are let inside the dome, when it opens, to queue outside the arena. If that's the case, a midday arrival may mean quite a pleasant GA line experience, out of the weather, inside the dome.
For what it's worth, I think I'll be one of those U2 crazies, camping out for the Nov 2 show.
I've been to a load of O2 gigs, you're right, the queues are all within the dome. There's also loads of restaurants and bars where you can have a drink or a bite to eat, and also a cinema...