It's odd that they haven't provided more details. I guess we'll find out at midnight.
My suspicion is that the reason it says "credit card required" even though it's a loan, is that they WILL reserve an iPhone for you tonight so you can pick it up when you go into the store, but in order to do that they have to put a hold on your credit card until they verify your eligibility for the loan. That's my guess. So that would seem to indicate that you CAN reserve a phone and pick it up.
If, on the other hand, there is no way to specify color/size/model when you make the appointment, I think it's more likely you'll have to place the order in-store and wait.
But either way I think you have to activate the phone with a plan in-store.