Love 'em even if a few games get scuttled from time to time.
Short answer: Yes, it IS frowned upon (and against Gen Con's policies) to purchase games before the hall opens, so you will likely have a rough time finding exhibitors willing to sell. That is not to say it is IMPOSSIBLE, but it will be difficult without having a personal connection at each booth. Worst case tho, you'll at least have a primo spot in line.
Almost every booth with a large retail presence lets the exhibitor access people line up before the hall opens and then purchase right as the doors open. You may or may not feel a few pangs of guilt doing it, but it's very common. Just check with the booth and ask if they mind if you wait to make a purchase when the doors open.