I have not heard of any people trying walk-ins downtown. I very much doubt TSA would let a person in for the CBP post security office (you could always try a fully refundable ticket, or if you have UNITED lounge access you can get a pass even without a ticket).

FWIW to others reading the thread, there was a recent Hou Chronicle article noting CBP will come to an employers office if 15 people in the office apply and then do the interviews and prints with their portable print reader.