S.C.
From what I know about piercings in general, you shouldn't do anything to the area before it's pierced. If you put something on it like Orajel, the needle would push the Orajel through the hole. That could cause an infection. You don't want that! You'll have to rinse with Listerine anyway and that would most likely make the Orajel inaffective. I don't really think there's anything you can do except just do it. Piercing through scar tissue hurts like HELL. I had it done on my belly button. You could call the person that's going to do it and see what they say.
Absolutely do NOT coat the barbell with ANYTHING before you put it in your tongue. That would get inside the piercing and cause and infection.
And for future dental appointments, get a plastic barbell. It won't affect the x-rays.