Here is the general page that lets you search for regulations.
http://www.access.gpo.gov/nara/cfr/index.html
You can follow the links to get to Title 49, Chapter 5 NHTSA/DOT, which will take you to Part 571 Standard 301. Reading the standard, it would appear not to directly implicate gravity feed systems, however, as is with all federal regulations they are interpereted to apply to a given application. These regs apply to passenger vehicles and trucks under 10k. There are some gravity feed systems which can be used on certain vehicle types (ICC Diesel systems for example). The purpose of this and most engineering regulations is to spell out a standard which vehicles must meet. Any modifications must at a minimum meet OEM standards. The regulations dictate in this case, fuel system designs, which is why OEM tanks do not feed from the bottom, for example. Ever wonder why sending units are in the top of a tank and are required to draw up out of a tank? Regulations make it so, because any fuel system breach should not allow the contents of a tank to drain. It would be easier to have sending units at the bottom of a tank, however that would be a failure point subject to the forces of gravity. Manufacturers design the fuel systems to retain the fuel in the tank in most situations, barring an actual puncture of the tank itself. Cutting the suppy line of my Aux tank, or breaking off any component up to the tank will only drain the fuel in the line itself, since the sending unit is on the top, and draws fuel out. I'm sure with your system, which you say has a valve at the tank, would not leak out any more fuel than in the line either, however, if the valve failed, it could discharge the entire contents of the tank due to its gravity feed design. It is this design difference, which per se makes a gravity feed system unable to meet at a minimum OEM standards, which makes it illegal. Now that is, exactly, what is referred to as legal mumbo jumbo. But thats how it works. Not to mention the other design requirements which must be met, such as a rollover valve, etc, that aftermarket manufacturers have already designed into their systems. But Federal Motor Vehicle Safety Standards aren't the only regulations you have to worry about, theres also VESC-22, NFPA 1192, ANSI 119.2, EPA, and State regulations which would also come into play, as States all impose regulations that mirror Federal laws.
Laws which were put in place to avoid having millions of 75 Ford Pinto clones on the road that burst into flames after every fenderbender. But, with my senseless rant almost finished, it really doesn't mean much in your case, because unless you become overcome with waves of unbearable guilt and drive your shiny DMAX over to the nearest State Police Post and turn yourself in, no one will be the wiser. If your still wondering where your setup fits into the big picture, you can email the appropriate agency and get a clarification. Anyways, thats my rant and I'm stickin to it!
Mike