Jamo,
Did your client get cited for importing a vehicle with non-DOT tires?
I am far form an expert on this, but I did stay at a Holiday Inn Express once...
My understanding is that the federal government has no control over highways. That is why they could not impose a 55 MPH speed limit years ago. They instead withheld a state’s federal monies until the state adopted the 55 limit.
They can require, via commerce controls, that all vehicles that are sold or imported meet Federal Motor Vehicle Safety standards. Also, any equipment installed for compensation (i.e. you get paid for installing) that is to be used on highways must met safety standards.
So, Discount Tire cannot mount non-DOT tires on your road car. Neither can Midas Muffler install non catalytic exhaust systems nor the Ford dealer install a hot Crower cam. Instead, state laws govern what pollution limits you must meet and what tire standards you use on the highways. So you personally can install any exhaust/engine components you want as long as it passes your state inspection.
States could enact laws that specify DOT approved tires, but as far as I know, my state just stipulates a minimum tread depth. If I mount non-DOT tires myself and they have the required tread depth, I am good to go.
However, like you say, I could be full of
it 
.
cheers,