Slightly related but where would someone park if they wanted to line up at 0700? And what if they had a firearm with them if they wanted to register the same day?
Depends. Long guns are different from handguns.
Note: I would not leave any firearms in the car while you are not in it. If that means lugging them to the line to get a time slip and then go back to the car only to return at a later time, it is what it is. Not much you can do about that.
Long guns: You can be in possession of the guns to register. Just need your permit to acquire & ID.
Handguns: You have 2 options --
(1) Meet the seller at HPD so he can be in possession of the handgun/s until he signs your permit. Once he signs the permit and hands you the gun/s, he can leave. Next step is to register.
(2) Pick up the permit, then drive to the seller's location to pick up the gun/s. Whether it's at a gun store, the seller's home, or a mutually agreed upon location, the permit signing is the same. Seller signs permit, gives you the gun/s, then you return to HPD to register the gun/s.
I went with my Son-in-Law the day his permit was ready to transfer 2 of my handguns to him. I carried the guns in about 12:30. He picked up the permit, I signed it, gave him the guns, and we walked into the registration vestibule.
I showed him how to use the clearing bin, the officer started the registration process, and we left with my SIL carrying HIS handguns.
Took less than 20 minutes to pick up the permit, have me sign it, and register under his name. There was nobody ahead of us for registration.
If I had to offer advice, I'd say don't go at 0700 unless you really need to be at work or somewhere else on time. Go later in the day. All the early registration walk-ins will be done, and you can actually do a "walk-in registration".
For what it's worth. YMMV.