I'm thinking you're allowed to do whatever you want. Even when it comes to soft openings typically they allow guests to do whatever they want and wait as much as they want right outside the queue even though there is no guaranteed, or even a known fact that the ride won't open. I would imagine the same would be applied to a ride that is officially open.
However, whether you can actually wait in the queue itself, or not, you'll probably have to ask a TM outside the entrance of the ride, however if you want to wait right outside the ride first thing in the morning, or wait around the entrance during mid-day that's your decision. I'm sure someone who has been there can officially tell whether they'll allow people to actually line up, or not.
The mid-day announcement is done probably not only make people aware it's not going to open until mid-day, which could be anytime in the very late morning, or early afternoon, but more likely probably to discourage people from waiting around outside the queue as well.