I read the below as you can install a tank drain/sampling port and fuel cell but everything between the tank and the rubber lines that connect to the fuel rail must remain stock.
Wouldn't the best place to put a drain be at the back of the car next to the pump like you are thinking? I think the most convenient place would be under the hood, but Id rather not open myself up to a faulty valve venting under the hood.
13.6 Fuel Delivery System
All components of the fuel delivery system must remain stock and unmodified, except for the
following.
13.6.1 The stock fuel tank may be replaced with a fuel cell(s) conforming to the NASA CCR’s,
located in the rear of the car no farther forward than the forward edge of the stock tank. The
maximum capacity of the fuel cell system is 21.1 gallons.
13.6.2 Any fuel cap may be used.
13.6.3 A fuel sampling port maybe added.
13.6.4 A fuel tank drain system maybe added.
13.6.5 Rubber fuel lines from the chassis to the fuel rail may be replaced or modified. Any
covering or heat shielding allowed on these lines in the interest of fire prevention.