Anybody know what motels don't requiere the girls to register? My girl isn't a pro and wants to be discrete.

If you are already are in the room or if you arrive together.  

Pueblo Amigo is pretty easy for the ladies to get to the room without visiting with the reception.  You could also meet her in the lobby at either Grand, Hyatt Place, or Marriott.  I don't believe Caesars asks the chicas for ID as well.

Last time I stayed at the Marriott I met the girl in the lobby and they made her sign in.

That happens sometimes.

If you arrive with the girl many "love motels" do not require the girl to register.  As stated above, La Mansion is one.  El Parisio (on 9th Street) is another.  IME a hobbyist doesn't even have to show ID to rent a room there.  (It comes with very secure garage parking and with in-and-out privileges.  Leave your room locked if you leave and are going to return.  They buzz you in and out of the main door and you leave the key with them until you return.  It's not as nice as the Ticuan, but much less expensive plus it has great anonymity.)

I have stayed at hotels (such as the Lucerna, which isn't escort friendly) where I register and give a fictitious name for a chica.  Then I meet the chica outside the hotel and since I already have the room key we just go to the room with the desk staff assuming she is the girl who is registered.  I suspect you could also do something similar at the Baja Inn across from Papas because you can go to the room without having to go past the front desk.  (It would be very easy to meet at Pampas then cross the street together and go to your room.)